nfscl: Move release of the clientID lock into nfscl_doclose()

This patch moves release of the shared clientID lock from nfsrpc_close()
just after the nfscl_doclose() call to the end of nfscl_doclose() call.
This does make the code cleaner, since the shared lock is acquired at
the beginning of nfscl_doclose(). The only semantics change is that
the code no longer drops and reaquires the NFSCLSTATELOCK() mutex,
which I do not believe will have a negative effect on the NFSv4 client.

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