compat: test and cocci patch for fs_dax_get_by_bdev

The fs_dax_get_by_bdev helper only appeared in 2017, before there was
fs_dax_get_by_host. Since fs_dax_get_by_bdev at the time of the
transition was almost trivially based on fs_dax_get_by_host, just open
code the function.

drbd/drbd-kernel-compat/cocci/fs_dax_get_by_bdev__no_present.cocci

@@ -0,0 +1,8 @@
+@@
+expression b;
+identifier d;
+@@
+- d = fs_dax_get_by_bdev(b);
++ if (!blk_queue_dax(b ->bd_queue))
++	return -ENODEV;
++ d = fs_dax_get_by_host(b->bd_disk->disk_name);

drbd/drbd-kernel-compat/gen_patch_names.c

@@ -468,6 +468,10 @@ int main(int argc, char **argv)
 	patch(1, "bio_max_vecs", true, false,
 	      COMPAT_HAVE_BIO_MAX_VECS, "present");
 
+	patch(1, "fs_dax_get_by_bdev", true, false,
+	      COMPAT_HAVE_FS_DAX_GET_BY_BDEV, "present");
+
+
 /* #define BLKDEV_ISSUE_ZEROOUT_EXPORTED */
 /* #define BLKDEV_ZERO_NOUNMAP */
 

drbd/drbd-kernel-compat/tests/have_fs_dax_get_by_bdev.c

@@ -0,0 +1,9 @@
+/* { "version": "v4.13-rc7", "commit": "78f35473508118df5ea04b9515ac3f1aaec0a980", "comment": "fs_dax_get_by_bdev was introduced", "author": "Dan Williams <dan.j.williams@intel.com>", "date": "Wed Aug 30 09:16:38 2017 -0700" } */
+
+#include <linux/blkdev.h>
+#include <linux/dax.h>
+
+struct dax_device *foo(struct block_device *bdev)
+{
+	return fs_dax_get_by_bdev(bdev);
+}