Rusty found on lguest with trim_bios_range, max_pfn is not right anymore, and
looks e820_remove_range does not work right.

[    0.000000] BIOS-provided physical RAM map:
[    0.000000]  LGUEST: 0000000000000000 - 0000000004000000 (usable)
[    0.000000] Notice: NX (Execute Disable) protection missing in CPU or disabled in BIOS!
[    0.000000] DMI not present or invalid.
[    0.000000] last_pfn = 0x3fa0 max_arch_pfn = 0x100000
[    0.000000] init_memory_mapping: 0000000000000000-0000000003fa0000

root cause is: the e820_remove_range doesn't handle the all covered
case.  e820_remove_range(BIOS_START, BIOS_END - BIOS_START, ...)
produces a bogus range as a result.

Make it match e820_update_range() by handling that case too.
Reported-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Tested-by: Rusty Russell <rusty@rustcorp.com.au>
LKML-Reference: <4BB18E55.6090903@kernel.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>