s390/kdump: rework invocation of DAT-off code

Calling kdump kernel is a two-step process that involves
invocation of the purgatory code: first time - to verify
the new kernel checksum and second time - to call the new
kernel itself.

The purgatory code operates on real addresses and does not
expect any memory protection. Therefore, before the purgatory
code is entered the DAT mode is always turned off. However,
it is only restored upon return from the new kernel checksum
verification. In case the purgatory was called to start the
new kernel and failed the control is returned to the old
kernel, but the DAT mode continues staying off.

The new kernel start failure is unlikely and leads to the
disabled wait state anyway. Still that poses a risk, since
the kernel code in general is not DAT-off safe and even
calling the disabled_wait() function might crash.

Introduce call_nodat() macro that allows entering DAT-off
mode, calling an arbitrary function and restoring DAT mode
back on. Switch all invocations of DAT-off code to that
macro and avoid the above described scenario altogether.

Name the call_nodat() macro in small letters after the
already existing call_on_stack() and put it to the same
header file.

Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
[hca@linux.ibm.com: some small modifications to call_nodat() macro]
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>