fusefs: fix page fault triggered by async notification when unmounted

A FUSE daemon can send asynchronous notification to the kernel in order
to, for example, invalidate an inode's cache. Fix a page fault that can
happen if the file system isn't yet mounted, or is already unmounted,
when that notification arrives.

PR: 290519
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D53356

(cherry picked from commit 5d42c8813976af484fd3e9a896c7839ba27ca764)