fusefs: VOP_FSYNC should be synchronous -- sometimes

I committed too hastily in r345390. There are cases, not directly reachable
from userland, where VOP_FSYNC ought to be asynchronous. This commit fixes
fusefs to handle VOP_FSYNC synchronously if and only if the VFS requests it.

PR: 236474
X-MFC-With: 345390
Sponsored by: The FreeBSD Foundation