Revert grabbing the socket lock in kqueue(2) filters.

Revert grabbing the socket lock in kqueue(2) filters.

This change exposed or created a situation where a CPU started to be
irresponsive while holding the KERNEL_LOCK().  These led to lockups and
even with MP_LOCKDEBUG it was not clear what happened to this CPU.

These situations have been experience by dhill@ with dcrwallet and jcs@
with syncthing.  Both applications are written in Go and do kevent(2)
& networking across multiple threads.