btrfs: use cond_resched_lock where possible

Clean the opencoded variant, cond_resched_lock also checks the lock for
contention so it might help in some cases that were not covered by
simple need_resched().

Signed-off-by: David Sterba <dsterba@suse.cz>
diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index e409025..1e34bc0 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -273,11 +273,7 @@
 		defrag = rb_entry(node, struct inode_defrag, rb_node);
 		kmem_cache_free(btrfs_inode_defrag_cachep, defrag);
 
-		if (need_resched()) {
-			spin_unlock(&fs_info->defrag_inodes_lock);
-			cond_resched();
-			spin_lock(&fs_info->defrag_inodes_lock);
-		}
+		cond_resched_lock(&fs_info->defrag_inodes_lock);
 
 		node = rb_first(&fs_info->defrag_inodes);
 	}