ubifs: Convert xattr inum to host order

UBIFS stores inode numbers as LE64 integers.
We have to convert them to host oder, otherwise
BE hosts won't be able to use the integer correctly.

Reported-by: kbuild test robot <lkp@intel.com>
Fixes: 9ca2d732 ("ubifs: Limit number of xattrs per inode")
Signed-off-by: Richard Weinberger <richard@nod.at>