nfscl: Change the cred argument to non-NULL for pNFS proxies

Commit 326bcf9394c7 added a "cred" argument to nfscl_reqstart().
For the pNFS proxy calls on the server, the argument
should be "cred" instead of NULL.
This patch fixes this.

Since the argument is not yet used, this patch
should not result in a semantics change.

PR: 260011
MFC after: 2 weeks