x86: add table start and end sanity checks to 32-bit init_memory_mapping()

Impact: cleanup

This patch adds a sanity check to the 32-bit version of
init_memory_mapping() to reduce the diff to the 64-bit version.

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Yinghai Lu <yinghai@kernel.org>
LKML-Reference: <1236257708-27269-9-git-send-email-penberg@cs.helsinki.fi>
Signed-off-by: Ingo Molnar <mingo@elte.hu>