Fix KSTACK_PAGES issue when the default value was changed in KERNCONF

If KSTACK_PAGES was changed to anything alse than the default,
the value from param.h was taken instead in some places and
the value from KENRCONF in some others. This resulted in
inconsistency which caused corruption in SMP envorinment.

Ensure all places where KSTACK_PAGES are used the opt_kstack_pages.h
is included.

The file opt_kstack_pages.h could not be included in param.h
because was breaking the toolchain compilation.

Reviewed by: kib
Obtained from: Semihalf
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D3094