fifos: delegate unhandled kqueue filters to underlying filesystem

This gives the vfs layer a chance to provide handling for EVFILT_VNODE,
for instance. Change pipe_specops to use the default vop_kqfilter to
accommodate fifoops that don't specify the method (i.e. all in-tree).

Based on a patch by Jan Kokemüller.

PR: 225934
Reviewed by: kib, markj (both pre-KASSERT)
Differential Revision: https://reviews.freebsd.org/D32271