NFS - Use mutex API, begin refactoring the state machine.

NFS - Use mutex API, begin refactoring the state machine.

* Use the mtx_*() API instead of roll-your-own locks for the send and
  receive locks.

* Refactor nfs_request().  Break the procedure up into multiple pieces
  for upcoming nfsiod/nfsd work (as in: getting rid of them).

  The main thing here is to split off the 'setup', 'send', 'receive',
  and 'process reply' parts.  This will make it easier to construct
  a kernel thread to i.e. just do the 'setup/send' part, and another
  to do the 'receive/reply' part.