vos: Avoid LWP stack overflow error on SIGINT

vos: Avoid LWP stack overflow error on SIGINT

vsproc's method of longjmp'ing out of a signal handler can thoroughly
confuse LWP. At the very least, the information on our stack becomes
incorrect, since we're no longer using the stack supplied by LWP. At the
very least, we seem to (almost) always fail the sanity check checking
for stack overflow. To prevent the test from happening, NULL the stack
pointer before the longjmp so we can attempt recovery.

FIXES 33360

Change-Id: Ie910cb1cc8564ac119c3b751fd8295bf312a3ce7
Reviewed-on: http://gerrit.openafs.org/1559
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>