[S390] Bad kexec control page allocation.

KEXEC_CONTROL_MEMORY_LIMIT is an unsigned long value and therefore
should be defined as one. Otherwise the kexec control page can be
allocated above 2GB which will cause a specification exception on the
sam31 instruction in the s390 kexec relocation code.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>