In principle, inotify can be used to leak timing information about I/O
device accesses by privileged process, which might be sensitive, e.g.,
in the case of /dev/input. See https://inoti.fyi/#1-linux-inter-keystroke-timing
In practice this does not work since I/O to devfs nodes does not go
through vnodes, so the INOTIFY() calls in various VOP hooks do not fire.
But, let's explicitly define a no-op VOP_INOTIFY to help ensure that no
one tries to "fix" this bug.
Alternately we could simply ignore VCHR/VBLK vnodes in vn_inotify().
However, maybe someday it'll be desirable to enable inotify for specific
device files, in which case the filtering should live in devfs.
Add a regression test.
Reported by: Sudheendra Raghav Neela <sneela@tugraz.at>