uidgid: make uid_valid and gid_valid work with !CONFIG_MULTIUSER

{u,g}id_valid call {u,g}id_eq, which calls __k{u,g}id_val on both
arguments and compares.  With !CONFIG_MULTIUSER, __k{u,g}id_val return a
constant 0, which makes {u,g}id_valid always return false.  Change
{u,g}id_valid to compare their argument against -1 instead.  That produces
identical results in the normal CONFIG_MULTIUSER=y case, but with
!CONFIG_MULTIUSER will make {u,g}id_valid constant-fold into "return
true;" rather than "return false;".

This fixes uses of devpts without CONFIG_MULTIUSER.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>,
Cc: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/uidgid.h

@@ -109,12 +109,12 @@ static inline bool gid_lte(kgid_t left, kgid_t right)
 
 static inline bool uid_valid(kuid_t uid)
 {
-	return !uid_eq(uid, INVALID_UID);
+	return __kuid_val(uid) != (uid_t) -1;
 }
 
 static inline bool gid_valid(kgid_t gid)
 {
-	return !gid_eq(gid, INVALID_GID);
+	return __kgid_val(gid) != (gid_t) -1;
 }
 
 #ifdef CONFIG_USER_NS