fs: btrfs: simplify close_ctree_fs_info()

At the beginning of close_ctree_fs_info() the value 0 is assigned to err
and never changed before testing it.

Let's get rid of the superfluous variable.

Fixes: f06bfcf5 ("fs: btrfs: Crossport open_ctree_fs_info() from btrfs-progs")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Qu Wenruo <wqu@suse.com>