Make stack grow use the same gap as stack create.

Store stack_guard_page * PAGE_SIZE into the gap->next_read field at
the time of the stack creation. This makes the used guard size
consistent between stack creation and stack grow time.

Suggested by: alc
Reviewed by: alc, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D21384