CRED: Fix NFSD regression

Fix a regression in NFSD's permission checking introduced by the credentials
patches.  There are two parts to the problem, both in nfsd_setuser():

 (1) The return value of set_groups() is -ve if in error, not 0, and should be
     checked appropriately.  0 indicates success.

 (2) The UID to use for fs accesses is in new->fsuid, not new->uid (which is
     0).  This causes CAP_DAC_OVERRIDE to always be set, rather than being
     cleared if the UID is anything other than 0 after squashing.

Reported-by: J. Bruce Fields <bfields@fieldses.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: James Morris <jmorris@namei.org>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>

fs/nfsd/auth.c

@@ -76,10 +76,10 @@ int nfsd_setuser(struct svc_rqst *rqstp, struct svc_export *exp)
 
 	ret = set_groups(new, gi);
 	put_group_info(gi);
-	if (!ret)
+	if (ret < 0)
 		goto error;
 
-	if (new->uid)
+	if (new->fsuid)
 		new->cap_effective = cap_drop_nfsd_set(new->cap_effective);
 	else
 		new->cap_effective = cap_raise_nfsd_set(new->cap_effective,