nfs: kuid and kgid conversions for nfs/inode.c

- Use uid_eq and gid_eq when comparing kuids and kgids.
- Use make_kuid(&init_user_ns, -2) and make_kgid(&init_user_ns, -2) as
  the initial uid and gid on nfs inodes, instead of using the typeunsafe
  value of -2.

Cc: "J. Bruce Fields" <bfields@fieldses.org>
Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>