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>