Add size restriction to the kdump documentation

In efi_init() memory aligns in IA64_GRANULE_SIZE(16M). If set "crashkernel=1024M-:600M"
and use sparse memory model, when crash kernel booting it changes [128M-728M] to [128M-720M].

But initrd memory is in [709M-727M], and virt_addr_valid() *can not* check the invalid pages
when freeing initrd memory, because there are some pages missed at the end of the section,
and this causes error.

...
Unpacking initramfs...
Freeing initrd memory: 19648kB freed
BUG: Bad page state in process swapper  pfn:02d00
page:e0000000102dd800 flags:(null) count:0 mapcount:1 mapping:(null) index:0

Call Trace:
 [<a000000100018dc0>] show_stack+0x80/0xa0
                                sp=e000000021e8fbd0 bsp=e000000021e81360
 [<a00000010090fcc0>] dump_stack+0x30/0x50
                                sp=e000000021e8fda0 bsp=e000000021e81348
 [<a0000001001a3180>] bad_page+0x280/0x380
                                sp=e000000021e8fda0 bsp=e000000021e81308
 [<a0000001001a8740>] free_ho...