fusefs: fix two bugs regarding fcntl file locks

fusefs: fix two bugs regarding fcntl file locks

1) F_SETLKW (blocking) operations would be sent to the FUSE server as
   F_SETLK (non-blocking).

2) Release operations, F_SETLK with lk_type = F_UNLCK, would simply
   return EINVAL.

PR:		253500
Reported by:	John Millikin <jmillikin@gmail.com>

(cherry picked from commit 929acdb19acb67cc0e6ee5439df98e28a84d4772)

fusefs: fix a dead store in fuse_vnop_advlock

kevans actually caught this in the original review and I fixed it, but
then I committed an older copy of the branch.  Whoops.

Reported by:	kevans
Differential Revision:	https://reviews.freebsd.org/D29031

(cherry picked from commit 9c5aac8f2e84ca4bbdf82514302c08c0453ec59b)