nfscl: Fix handling of nd_slotid while handling NFSERR_BADSESSION

When the NFSv4.1/4.2 client is handling a server error
of NFSERR_BADSESSION, it retries RPCs with a new session.
Without this patch, the nd_slotid was not being updated
for the new session.

This would result in a bogus console message like
"Wrong session srvslot=X slot=Y" and then it would
free the incorrect slot, often generating a
"freeing free slot!!" console message as well.

This patch fixes the problem.

Note that FreeBSD NFSv4.1/4.2 servers only
generate a NFSERR_BADSESSION error after a reboot
or after a client does a DestroySession operation.

PR: 260011
MFC after: 1 week