x86: Dump filtering supports x86_64 sparsemem

This patch adds the symbol "init_level4_pgt" to the vmcoreinfo data so
that makedumpfile (dump filtering command) supports x86_64 sparsemem 
kernel of linux-2.6.24.

makedumpfile creates a small dumpfile by excluding unnecessary pages for
the analysis. It checks attributes in page structures and distinguishes
necessary pages and unnecessary ones. To check them, makedumpfile gets
the vmcoreinfo data which has the minimum debugging information only for
dump filtering.

For older x86_64 kernel (linux-2.6.23 or before), makedumpfile translates
the virtual address of page structure into physical address by subtracting
PAGE_OFFSET from virtual address, but this translation isn't effective for
linux-2.6.24 sparsemem kernel, because its page structures are in virtual
memmap area. makedumpfile should translate their virtual address by 4-levels
paging and it needs the symbol "init_level4_pgt".

Signed-off-by: Ken'ichi Ohmichi <oomichi@mxs.nes.nec.co.jp>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>