MFC r352404, r352413-r352414

r352404:
fusefs: fix some minor issues with fuse_vnode_setparent

• When unparenting a vnode, actually clear the flag. AFAIK this is basically a no-op because we only unparent a vnode when reclaiming it or when unlinking.

• There's no need to call fuse_vnode_setparent during reclaim, because we're about to free the vnode data anyway.

Reviewed by: emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21630

r352413:
fusefs: fix some minor Coverity CIDs in the tests

Where open(2) is expected to fail, the tests should assert or expect that
its return value is -1. These tests all accepted too much but happened to
pass anyway.

Reported by: Coverity
Coverity CID: 1404512, 1404378, 1404504, 1404483
Sponsored by: The FreeBSD Foundation

r352414:
fusefs: initialize C++ classes the Coverity way

Coverity complained that I wasn't initializing some class members until the
SetUp method. Do it in the constructor instead.

Reported by: Coverity
Coverity CIDs: 1404352, 1404378
Sponsored by: The FreeBSD Foundation