btrfs: don't hold CPU for too long when defragging a file

There is a user report about "btrfs filesystem defrag" causing 120s
timeout problem.

For btrfs_defrag_file() it will iterate all file extents if called from
defrag ioctl, thus it can take a long time.

There is no reason not to release the CPU during such a long operation.

Add cond_resched() after defragged one cluster.

CC: stable@vger.kernel.org # 5.16
Link: https://lore.kernel.org/linux-btrfs/10e51417-2203-f0a4-2021-86c8511cc367@gmx.com
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ioctl.c

@@ -1629,6 +1629,7 @@ int btrfs_defrag_file(struct inode *inode, struct file_ra_state *ra,
 			ret = 0;
 			break;
 		}
+		cond_resched();
 	}
 
 	if (ra_allocated)