When growing the state, also grow the seed array. Otherwise memory

When growing the state, also grow the seed array. Otherwise memory
that was not allocated will be accessed.

This necessitated refactoring state seed allocation from
ipf_state_soft_init() into a new common ipf_state_seed_alloc() function
as it is now also used by ipf_state_rehash() when changing the size of
the state hash table in addition to by ipf_state_soft_init() during
initialization.

According to Christos Zoulas <christos@NetBSD.org>:

The bug was encountered by a NetBSD vendor who's customer machines had
large ipfilter states. The bug was reliably triggered by resizing the
state variables using "ipf -T".

Submitted by:	Christos Zoulas <christos@NetBSD.org>
Reviewed by:	delphij, rgrimes
Obtained from:	NetBSD ip_state.c CVS revs r1.9 and r1.10
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D13755