NFSD: callback request does not use correct credential for AUTH_SYS

Currently callback request does not use the credential specified in
CREATE_SESSION if the security flavor for the back channel is AUTH_SYS.

Problem was discovered by pynfs 4.1 DELEG5 and DELEG7 test with error:
DELEG5   st_delegation.testCBSecParms     : FAILURE
           expected callback with uid, gid == 17, 19, got 0, 0
Signed-off-by: Dai Ngo <dai.ngo@oracle.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Fixes: 8276c902 ("SUNRPC: remove uid and gid from struct auth_cred")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs4callback.c

@@ -946,8 +946,8 @@ static const struct cred *get_backchannel_cred(struct nfs4_client *clp, struct r
 		if (!kcred)
 			return NULL;
 
-		kcred->uid = ses->se_cb_sec.uid;
-		kcred->gid = ses->se_cb_sec.gid;
+		kcred->fsuid = ses->se_cb_sec.uid;
+		kcred->fsgid = ses->se_cb_sec.gid;
 		return kcred;
 	}
 }