btrfs: fix deadlock when writing out space cache

If we fail to prepare our pages for whatever reason (out of memory in
our case) we need to make sure to drop the block_group->data_rwsem,
otherwise hilarity ensues.
Signed-off-by: NJosef Bacik <jbacik@fb.com>
Reviewed-by: NOmar Sandoval <osandov@fb.com>
Reviewed-by: NLiu Bo <bo.li.liu@oracle.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
[ add label and use existing unlocking code ]
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/free-space-cache.c

@@ -1264,7 +1264,7 @@ static int __btrfs_write_out_cache(struct btrfs_root *root, struct inode *inode,
 	/* Lock all pages first so we can lock the extent safely. */
 	ret = io_ctl_prepare_pages(io_ctl, inode, 0);
 	if (ret)
-		goto out;
+		goto out_unlock;
 
 	lock_extent_bits(&BTRFS_I(inode)->io_tree, 0, i_size_read(inode) - 1,
 			 &cached_state);
@@ -1358,6 +1358,7 @@ static int __btrfs_write_out_cache(struct btrfs_root *root, struct inode *inode,
 out_nospc:
 	cleanup_write_cache_enospc(inode, io_ctl, &cached_state);
 
+out_unlock:
 	if (block_group && (block_group->flags & BTRFS_BLOCK_GROUP_DATA))
 		up_write(&block_group->data_rwsem);