x86: unify kernel_physical_mapping_init() call in init_memory_mapping()

Impact: cleanup

The 64-bit version of init_memory_mapping() uses the last mapped
address returned from kernel_physical_mapping_init() whereas the
32-bit version doesn't. This patch adds relevant ifdefs to both
versions of the function to reduce the diff between them.

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Yinghai Lu <yinghai@kernel.org>
LKML-Reference: <1236257708-27269-8-git-send-email-penberg@cs.helsinki.fi>
Signed-off-by: Ingo Molnar <mingo@elte.hu>