nfscl: Restructure nfscl_freeopen() slightly

This patch factors the unlinking of the nfsclopen structure out of
nfscl_freeopen() into a separate function called nfscl_unlinkopen().
It also adds a new argument to nfscl_freeopen() to conditionally do
the unlink. Since this new argument is always passed in as "true"
at this time, no semantics change should occur.

This is being done to prepare the code for a future patch that fixes
the case where an NFSv4.1/4.2 server replies NFSERR_DELAY to a Close
operation.

MFC after: 2 week