x86: check range in reserve_early()

Impact: cleanup

one 32-bit system reports:

BIOS-provided physical RAM map:
 BIOS-e820: 0000000000000000 - 000000000009fc00 (usable)
 BIOS-e820: 000000000009fc00 - 00000000000a0000 (reserved)
 BIOS-e820: 00000000000f0000 - 0000000000100000 (reserved)
 BIOS-e820: 0000000000100000 - 000000001c000000 (usable)
 BIOS-e820: 00000000ffff0000 - 0000000100000000 (reserved)
DMI 2.0 present.
last_pfn = 0x1c000 max_arch_pfn = 0x100000
kernel direct mapping tables up to 1c000000 @ 7000-c000
..
RAMDISK: 1bc69000 - 1bfef4fa
..
0MB HIGHMEM available.
448MB LOWMEM available.
  mapped low ram: 0 - 1c000000
  low ram: 00000000 - 1c000000
  bootmap 00002000 - 00005800
(9 early reservations) ==> bootmem [0000000000 - 001c000000]
  #0 [0000000000 - 0000001000]   BIOS data page ==> [0000000000 - 0000001000]
  #1 [0000001000 - 0000002000]    EX TRAMPOLINE ==> [0000001000 - 0000002000]
  #2 [0000006000 - 0000007000]       TRAMPOLINE ==> [0000006000 - 0000007000]
  #3 [0000400000 - 00009ed14c]    TEXT DATA BSS ==> [0000400000 - 00009ed14c]
  #4 [001bc69000 - 001bfef4fa]          RAMDISK ==> [001bc69000 - 001bfef4fa]
  #5 [00009ee000 - 00009f2000]    INIT_PG_TABLE ==> [00009ee000 - 00009f2000]
  #6 [000009f400 - 0000100000]    BIOS reserved ==> [000009f400 - 0000100000]
  #7 [0000007000 - 0000007000]          PGTABLE
  #8 [0000002000 - 0000006000]          BOOTMAP ==> [0000002000 - 0000006000]

Notice the strange blank PGTABLE entry.

The reason is init_pg_table is big enough, and zero range is called
with init_memory_mapping/reserve_early().

So try to check the range in reserve_early()

v2: fix the reversed compare
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: nickpiggin@yahoo.com.au
Cc: ink@jurassic.park.msu.ru
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/e820.c

@@ -858,6 +858,9 @@ void __init reserve_early_overlap_ok(u64 start, u64 end, char *name)
  */
 void __init reserve_early(u64 start, u64 end, char *name)
 {
+	if (start >= end)
+		return;
+
 	drop_overlaps_that_are_ok(start, end);
 	__reserve_early(start, end, name, 0);
 }