x86: split out common code into find_overlapped_early()

This patch clean up reserve_early() family functions by extracting the
common part of reserve_early(), free_early() and bad_addr() into
find_overlapped_early().

Signed-off-by: Huang Ying <ying.huang@intel.com>
Cc: andi@firstfloor.org
Cc: mingo@redhat.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>