Disable nullfs cacheing on top of fusefs

Nullfs cacheing can keep a large number of vnodes active. That results in
more active FUSE file handles, causing some FUSE servers to use extra
resources. Disable nullfs cacheing for fusefs, just like we already do for
NFSv4.

PR: 245688
Reported by: MooseFS FreeBSD Team <freebsd@moosefs.pro>
MFC after: 2 weeks