nfsd: idmap use u32 not uid_t as the intermediate type

u32 and uid_t have the same size and semantics so this change
should have no operational effect.  This just removes the WTF
factor when looking at variables that hold both uids and gids
whos type is uid_t.

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