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
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21630