x86/debug: Extend the lower bound of crash kernel low reservations

The following change in 2013:

  0212f91 ("x86: Add Crash kernel low reservation")

... introduced reserve_crashkernel_low(). This function is used to
reserve crash kernel memory either if crashkernel=size,low is given
on the command line or if the region reserved by reserve_crashkernel
is entirely above 4G.

reserve_crashkernel_low() tries to find a block of 'low_size' bytes.
But there seems to be no good reason to restrict the lower bound
of the range to 'low_size'.

Make memblock_find_in_range() search from the start of memory.

Signed-off-by: Jiri Bohac <jbohac@suse.cz>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/20170616161602.2r7birrf2y3ylv6v@dwarf.suse.cz
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/setup.c

@@ -503,7 +503,7 @@ static int __init reserve_crashkernel_low(void)
 			return 0;
 	}
 
-	low_base = memblock_find_in_range(low_size, 1ULL << 32, low_size, CRASH_ALIGN);
+	low_base = memblock_find_in_range(0, 1ULL << 32, low_size, CRASH_ALIGN);
 	if (!low_base) {
 		pr_err("Cannot reserve %ldMB crashkernel low memory, please try smaller size.\n",
 		       (unsigned long)(low_size >> 20));