btrfs: tree-checker: Refactor inode key check into seperate function

Inode key check is not as easy as several lines, and it will be called
in more than one location (INODE_ITEM check and
DIR_ITEM/DIR_INDEX/XATTR_ITEM location key check).

So here refactor such check into check_inode_key().  And add extra
checks for XATTR_ITEM.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>