x86: rename find_max_pfn() to propagate_e820_map()

this function doesnt just 'find' the max_pfn - it also has
other side-effects such as registering sparse memory maps.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>