Fix ACL checks for NFS kernel server

For Linux NFS kernel server ops, fsuid and fsgid in
cred are populated with ids that operation is
being performed as, but euid and egid remain 0.

In Linux when setresuid(2) and setresgid(2) are
called, the fsuid and fsgid are set to the euid
and egid respectively.

This PR changes ZFS ACL checks to evaluate
fsuid / fsgid rather than euid / egid to avoid
accidentally granting elevated permissions to
NFS clients.

Signed-off-by: Andrew Walker <awalker@ixsystems.com>

module/os/linux/spl/spl-cred.c

@@ -128,7 +128,7 @@ groupmember(gid_t gid, const cred_t *cr)
 uid_t
 crgetuid(const cred_t *cr)
 {
-	return (KUID_TO_SUID(cr->euid));
+	return (KUID_TO_SUID(cr->fsuid));
 }
 
 /* Return the real user id */
@@ -156,7 +156,7 @@ crgetfsuid(const cred_t *cr)
 gid_t
 crgetgid(const cred_t *cr)
 {
-	return (KGID_TO_SGID(cr->egid));
+	return (KGID_TO_SGID(cr->fsgid));
 }
 
 /* Return the real group id */