Allow EVFILT_VNODE events to work on every filesystem type, not just
UFS by:

• Making the pre and post hooks for the VOP functions work even when

DEBUG_VFS_LOCKS is not defined.

• Moving the KNOTE activations into the corresponding VOP hooks.
• Creating a MNTK_NOKNOTE flag for the mnt_kern_flag field of struct

mount that permits filesystems to disable the new behavior.

• Creating a default VOP_KQFILTER function: vfs_kqfilter()

My benchmarks have not revealed any performance degradation.

Reviewed by: jeff, bde
Approved by: rwatson, jmg (kqueue changes), grehan (mentor)