MFi386: correctly calculate the top-of-stack when a kthread is created with a larger kernel stack. Remove inclusion of opt_kstack_pages.h now that it's unused.
MFi386: correctly calculate the top-of-stack when a kthread is created
with a larger kernel stack. Remove inclusion of opt_kstack_pages.h now
that it's unused.

Note: If anyone's toes got stepped on by me doing this let me know
privately please.

Approved by:	rwatson (mentor)

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