NFSv4: Cleanup: pass the nfs_open_context to nfs4_do_open

All the callers have an open_context at this point, and since we always
need one in order to do state recovery, it makes sense to use it as the
basis for the nfs4_do_open() call.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>