Allocate memory when dumping pipes with M_WAITOK flag.

Allocate memory when dumping pipes with M_WAITOK flag.
On a system with huge number of pipes, M_NOWAIT failes almost always,
because of memory fragmentation.
My fix is different than the patch proposed by Pawel Malachowski,
because in FreeBSD 5.x we cannot sleep while holding dummynet mutex
(in 4.x there is no such lock).
My fix is also ugly, but there is no easy way to prepare nice and clean fix.

PR:		kern/46557
Submitted by:	Eugene Grosbein <eugen@grosbein.pp.ru>
Reviewed by:	mlaier