block: generalize BlockLimits handling to cover bdrv_aio_discard too

bdrv_co_discard is only covering drivers which have a .bdrv_co_discard()
implementation, but not those with .bdrv_aio_discard(). Not very nice,
and easy to avoid.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block.c

@@ -4302,6 +4302,8 @@ static void coroutine_fn bdrv_discard_co_entry(void *opaque)
 int coroutine_fn bdrv_co_discard(BlockDriverState *bs, int64_t sector_num,
                                  int nb_sectors)
 {
+    int max_discard;
+
     if (!bs->drv) {
         return -ENOMEDIUM;
     } else if (bdrv_check_request(bs, sector_num, nb_sectors)) {
@@ -4317,55 +4319,55 @@ int coroutine_fn bdrv_co_discard(BlockDriverState *bs, int64_t sector_num,
         return 0;
     }
 
-    if (bs->drv->bdrv_co_discard) {
-        int max_discard = bs->bl.max_discard ?
-                          bs->bl.max_discard : MAX_DISCARD_DEFAULT;
+    if (!bs->drv->bdrv_co_discard && !bs->drv->bdrv_aio_discard) {
+        return 0;
+    }
 
-        while (nb_sectors > 0) {
-            int ret;
-            int num = nb_sectors;
+    max_discard = bs->bl.max_discard ?  bs->bl.max_discard : MAX_DISCARD_DEFAULT;
+    while (nb_sectors > 0) {
+        int ret;
+        int num = nb_sectors;
 
-            /* align request */
-            if (bs->bl.discard_alignment &&
-                num >= bs->bl.discard_alignment &&
-                sector_num % bs->bl.discard_alignment) {
-                if (num > bs->bl.discard_alignment) {
-                    num = bs->bl.discard_alignment;
-                }
-                num -= sector_num % bs->bl.discard_alignment;
+        /* align request */
+        if (bs->bl.discard_alignment &&
+            num >= bs->bl.discard_alignment &&
+            sector_num % bs->bl.discard_alignment) {
+            if (num > bs->bl.discard_alignment) {
+                num = bs->bl.discard_alignment;
             }
+            num -= sector_num % bs->bl.discard_alignment;
+        }
 
-            /* limit request size */
-            if (num > max_discard) {
-                num = max_discard;
-            }
+        /* limit request size */
+        if (num > max_discard) {
+            num = max_discard;
+        }
 
+        if (bs->drv->bdrv_co_discard) {
             ret = bs->drv->bdrv_co_discard(bs, sector_num, num);
-            if (ret) {
-                return ret;
+        } else {
+            BlockDriverAIOCB *acb;
+            CoroutineIOCompletion co = {
+                .coroutine = qemu_coroutine_self(),
+            };
+
+            acb = bs->drv->bdrv_aio_discard(bs, sector_num, nb_sectors,
+                                            bdrv_co_io_em_complete, &co);
+            if (acb == NULL) {
+                return -EIO;
+            } else {
+                qemu_coroutine_yield();
+                ret = co.ret;
             }
-
-            sector_num += num;
-            nb_sectors -= num;
         }
-        return 0;
-    } else if (bs->drv->bdrv_aio_discard) {
-        BlockDriverAIOCB *acb;
-        CoroutineIOCompletion co = {
-            .coroutine = qemu_coroutine_self(),
-        };
-
-        acb = bs->drv->bdrv_aio_discard(bs, sector_num, nb_sectors,
-                                        bdrv_co_io_em_complete, &co);
-        if (acb == NULL) {
-            return -EIO;
-        } else {
-            qemu_coroutine_yield();
-            return co.ret;
+        if (ret) {
+            return ret;
         }
-    } else {
-        return 0;
+
+        sector_num += num;
+        nb_sectors -= num;
     }
+    return 0;
 }
 
 int bdrv_discard(BlockDriverState *bs, int64_t sector_num, int nb_sectors)