Make the extra stack re-entrant and "hop back" to check for irqs.

Make the extra stack re-entrant and "hop back" to check for irqs.

When on the extra stack and it's necessary to check for irqs, switch
back to the original caller's stack to check for irqs.  Make the extra
stack re-entrant, so that a new user of the extra stack wont collide
with an existing user.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>