userns: Implement struct kqid

Add the data type struct kqid which holds the kernel internal form of
the owning identifier of a quota.  struct kqid is a replacement for
the implicit union of uid, gid and project id stored in an unsigned
int and the quota type field that is was used in the quota data
structures.  Making the data type explicit allows the kuid_t and
kgid_t type safety to propogate more thoroughly through the code,
revealing more places where uid/gid conversions need be made.

Along with the data type struct kqid comes the helper functions
qid_eq, qid_lt, from_kqid, from_kqid_munged, qid_valid, make_kqid,
make_kqid_invalid, make_kqid_uid, make_kqid_gid.

Cc: Jan Kara <jack@suse.cz>
Cc: Dave Chinner <david@fromorbit.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>