kboot: use 128MB for the heap area, ZFS needs a lot of memory

ZFS uses a lot of memory. The old minimal allocations won't work when
ZFS support is added. Most environments this will be used (or will
liekly be used) have >> 256MB, 128MB should be safe everywhere and allow
examination of a fair number of ZFS pools to boot from.

Sponsored by: Netflix

(cherry picked from commit 4dd3e76881ad025170b0cfb5455680b3c89fe263)