x86: e820: user-defined memory maps: remove the range instead of update it to reserved

also let mem= to print out modified e820 map too

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Cc: Bernhard Walle <bwalle@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>