Since commit 998f5bbe ("s390/kasan: fix early pgm check handler
execution") early pgm check handler is executed with DAT on if Kasan
is enabled.

Still there is a window between setup_lowcore_dat_off() and
setup_lowcore_dat_on() when int handlers could be executed with DAT off
under Kasan. If this happens the kernel ends up in pgm check loop due
to Kasan shadow memory access attempts.

With Kasan enabled paging is initialized much earlier and DAT flag has to
be on at all times instrumented code is executed. Make sure int handlers
are set up to be called with DAT on right away in this case.
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>