s390/kexec: add __GFP_NORETRY to KEXEC_CONTROL_MEMORY_GFP

Avoid invoking the OOM-killer when allocating the control page. This
is the s390 variant of commit dc5cccac

 ("kexec: don't invoke
OOM-killer for control page allocation").

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>