s390x: skey.c: rework the interrupt handler

The skey test currently uses a cleanup function to work around the
issues that arise when the lowcore is not mapped, since the interrupt
handler needs to access it.

Instead of a cleanup function, simply disable DAT for the interrupt
handler for the tests that remap page 0. This is needed in preparation
of and upcoming patch that will cause the interrupt handler to read
from lowcore before calling the cleanup function.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Acked-by: Janosch Frank <frankja@linux.ibm.com>