UNRPC: Return specific error code on kmalloc failure

Although the callers of this function only care about whether the
return value is null or not, we should still give a rigorous
error code.

Smatch tool warning:
net/sunrpc/auth_gss/svcauth_gss.c:784 gss_write_verf() warn: returning
-1 instead of -ENOMEM is sloppy

No functional change, just more standardized.
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

net/sunrpc/auth_gss/svcauth_gss.c

@@ -781,7 +781,7 @@ gss_write_verf(struct svc_rqst *rqstp, struct gss_ctx *ctx_id, u32 seq)
 	svc_putnl(rqstp->rq_res.head, RPC_AUTH_GSS);
 	xdr_seq = kmalloc(4, GFP_KERNEL);
 	if (!xdr_seq)
-		return -1;
+		return -ENOMEM;
 	*xdr_seq = htonl(seq);
 
 	iov.iov_base = xdr_seq;