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
(cherry picked from commit 2b766d5e5a73fce7120131d56bffc1715f71e7c9)