nfscl: Add a cred argument to nfscl_reqstart()

To deal with broken session slots caused by the use of the
"soft" and/or "intr" mount options, nfsv4_sequencelookup()
will be modified to track the potentially broken session
slots. Then, when all session slots are potentially
broken, do a DeleteSession operation, so that the NFSv4
server will reply NFSERR_BADSESSION to uses of the session.
These changes will be done in future commits. However,
to do the DeleteSession RPC, a "cred" argument is needed
for nfscl_reqstart(). This patch adds this argument,
which is unused at this time. If the argument is NULL,
it indicates that DeleteSession should not be done
(usually because the RPC does not use sessions).

This patch should not cause any semantics change.

PR: 260011
MFC after: 2 weeks