fs: Add helper to clean bdev aliases under a bh and use it

Add a helper function that clears buffer heads from a block device
aliasing passed bh. Use this helper function from filesystems instead of
the original unmap_underlying_metadata() to save some boiler plate code
and also have a better name for the functionalily since it is not
unmapping anything for a *long* time.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>

fs/ntfs/aops.c

@@ -765,7 +765,7 @@ lock_retry_remap:
 			}
 			// TODO: Instantiate the hole.
 			// clear_buffer_new(bh);
-			// unmap_underlying_metadata(bh->b_bdev, bh->b_blocknr);
+			// clean_bdev_bh_alias(bh);
 			ntfs_error(vol->sb, "Writing into sparse regions is "
 					"not supported yet. Sorry.");
 			err = -EOPNOTSUPP;