Btrfs: get better concurrency for snapshot-aware defrag work

Using spinning case instead of blocking will result in better concurrency
overall.
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/inode.c

@@ -2312,6 +2312,7 @@ static noinline int relink_extent_backref(struct btrfs_path *path,
 	key.type = BTRFS_EXTENT_DATA_KEY;
 	key.offset = start;
 
+	path->leave_spinning = 1;
 	if (merge) {
 		struct btrfs_file_extent_item *fi;
 		u64 extent_len;
@@ -2368,6 +2369,7 @@ static noinline int relink_extent_backref(struct btrfs_path *path,
 
 	btrfs_mark_buffer_dirty(leaf);
 	inode_add_bytes(inode, len);
+	btrfs_release_path(path);
 
 	ret = btrfs_inc_extent_ref(trans, root, new->bytenr,
 			new->disk_len, 0,
@@ -2381,6 +2383,7 @@ static noinline int relink_extent_backref(struct btrfs_path *path,
 	ret = 1;
 out_free_path:
 	btrfs_release_path(path);
+	path->leave_spinning = 0;
 	btrfs_end_transaction(trans, root);
 out_unlock:
 	unlock_extent_cached(&BTRFS_I(inode)->io_tree, lock_start, lock_end,