Remove lock recursion for the pipe pair mutex, and disable the
recursion on mutex initialization.

The only places where the recursive acquire is performed are read and
write filters, since knlist, which uses the pipe pair mutex as lock,
is locked when filter is called.

The recursion was added in r93296, and consistent locking for
kn_fop->f_event() introduced in r133741.

Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 month