nfscl: Replace "cred" with NULL to cleanup code

Commit 326bcf9394c7 added a new "cred" argument to nfscl_reqstart().
Fsinfo is a NFSv3 RPC and since the "cred" argument is not
used for NFSv3, it does not matter what is passed in.
However, to be consistent with the rest of the patch, change the
argument to NULL.

This patch should not result in a semantics change.

PR: 260011
MFC after: 2 weeks