NLM: Fix arguments to NLM_CANCEL call

 The OpenGroup docs state that the arguments "block", "exclusive" and
 "alock" must exactly match the arguments for the lock call that we are
 trying to cancel.
 Currently, "block" is always set to false, which is wrong.

 See bug# 5956 on bugzilla.kernel.org.

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