block: preserve errno from fdatasync failures

When fdatasync() fails on a file backend we set a flag that
short-circuits any future attempts to call fdatasync(). The
first failure returns the true errno, but the later short-
circuited calls return a generic EIO. The latter is unhelpful
because fdatasync() can return a variety of errnos, including
EACCESS.

Reviewed-by: Connor Kuehl <ckuehl@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

block/file-posix.c

@@ -160,7 +160,7 @@ typedef struct BDRVRawState {
     bool discard_zeroes:1;
     bool use_linux_aio:1;
     bool use_linux_io_uring:1;
-    bool page_cache_inconsistent:1;
+    int page_cache_inconsistent; /* errno from fdatasync failure */
     bool has_fallocate;
     bool needs_alignment;
     bool drop_cache;
@@ -1333,7 +1333,7 @@ static int handle_aiocb_flush(void *opaque)
     int ret;
 
     if (s->page_cache_inconsistent) {
-        return -EIO;
+        return -s->page_cache_inconsistent;
     }
 
     ret = qemu_fdatasync(aiocb->aio_fildes);
@@ -1352,7 +1352,7 @@ static int handle_aiocb_flush(void *opaque)
          * Obviously, this doesn't affect O_DIRECT, which bypasses the page
          * cache. */
         if ((s->open_flags & O_DIRECT) == 0) {
-            s->page_cache_inconsistent = true;
+            s->page_cache_inconsistent = errno;
         }
         return -errno;
     }