ipsec: Clear pad bytes in PF_KEY messages

ipsec: Clear pad bytes in PF_KEY messages

Various handlers for SADB messages will allocate a new mbuf and populate
some structures in it.  Some of these structures, such as struct
sadb_supported, contain small reserved fields that are not initialized
and are thus leaked to userspace.

Fix the problem by adding a helper to allocate zeroed mbufs.  This
reduces code duplication and the overhead of zeroing these messages
isn't harmful.

Reviewed by:	zlei, melifaro
Reported by:	KMSAN
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D38068