nfscl: Move nfsrpc_destroysession into nfscommon

This patch moves nfsrpc_destroysession() into nfscommon.ko
and also modifies its arguments slightly. This will allow
the function to be called from nfsv4_sequencelookup() in
a future commit.

This patch should not result in a semantics change.

PR: 260011
MFC after: 2 weeks