btrfs: reserve correct number of items for unlink and rmdir

__btrfs_unlink_inode() calls btrfs_update_inode() on the parent
directory in order to update its size and sequence number. Make sure we
account for it.

Reviewed-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>