linprocfs: Add nodes for inotify

These map directly onto sysctls under vfs.inotify of the same name.

Reviewed by: olce, des
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D53142

(cherry picked from commit 28c0ef76841bbb322b6e39668e1c8e997c1f8000)