fusefs: support EVFILT_WRITE on /dev/fuse

/dev/fuse is always ready for writing, so it's kind of dumb to poll it.
But some applications do it anyway. Better to return ready than EINVAL.

Reviewed by: emaste, pfg
Differential Revision: https://reviews.freebsd.org/D30784

(cherry picked from commit 7b8622fa220b9c08041102f638f848c48e022644)

Simplify fuse_device_filt_write

It always returns 1, so why bother having a variable.

Pull Request: https://github.com/freebsd/freebsd-src/pull/478

(cherry picked from commit 9b876fbd504e5c718d8d0275b32d806ab14558c8)