x86/crash: Update the stale comment in reserve_crashkernel()

CRASH_KERNEL_ADDR_MAX has been missing for a long time,
update it with a more detailed explanation.
Signed-off-by: Xunlei Pang <xlpang@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Baoquan He <bhe@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Young <dyoung@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Robert LeBlanc <robert@leblancnet.us>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kexec@lists.infradead.org
Link: http://lkml.kernel.org/r/1485154103-18426-1-git-send-email-xlpang@redhat.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/setup.c

@@ -575,7 +575,9 @@ static void __init reserve_crashkernel(void)
 	/* 0 means: find the address automatically */
 	if (crash_base <= 0) {
 		/*
-		 *  kexec want bzImage is below CRASH_KERNEL_ADDR_MAX
+		 * Set CRASH_ADDR_LOW_MAX upper bound for crash memory,
+		 * as old kexec-tools loads bzImage below that, unless
+		 * "crashkernel=size[KMG],high" is specified.
 		 */
 		crash_base = memblock_find_in_range(CRASH_ALIGN,
 						    high ? CRASH_ADDR_HIGH_MAX