s390/crash: remove redundant panic() on save area allocation failure

Make save_area_alloc() return classic NULL on allocation failure.
The only caller smp_save_dump_cpus() does check the return value
already and panics if NULL is returned.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>