s390/ipl: fix physical-virtual confusion for diag308

Diag 308 subcodes expect a physical address as their parameter.

This currently is not a bug, but in the future physical and virtual
addresses might differ.

Fix the confusion by doing a virtual-to-physical conversion in the
exported diag308() and leave the assembly wrapper __diag308() alone.

Note that several callers pass NULL as addr, so check for the case when
NULL is passed and pass 0 to hardware since virt_to_phys(0) might be
nonzero.

Suggested-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Signed-off-by: Nico Boehr <nrb@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>