x86: use roundup() instead of PAGE_ALIGN() in find_early_table_space()

Impact: cleanup

This patch changes find_early_table_space() to use roundup() for rounding up
tables to page size to unify the common parts of the 32-bit and 64-bit
implementations.

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
LKML-Reference: <1236077705.2675.6.camel@penberg-laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>