s390/boot: Respect kaslr_enabled() for identity randomization

s390/boot: Respect kaslr_enabled() for identity randomization

CONFIG_RANDOMIZE_IDENTITY_BASE only enables support for randomizing the
identity mapping base. The randomization (identity base != 0) itself
should happen only when KASLR is enabled at runtime.

Guard the __identity_base update with kaslr_enabled() so nokaslr (and
other KASLR-disabled cases) keep the non-randomized identity mapping at 0.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>