kexec: fix Oops in crash_shrink_memory()

When crashkernel is not enabled, "echo 0 > /sys/kernel/kexec_crash_size"
OOPSes the kernel in crash_shrink_memory.  This happens when
crash_shrink_memory tries to release the 'crashk_res' resource which are
not reserved.  Also value of "/sys/kernel/kexec_crash_size" shows as 1,
which should be 0.

This patch fixes the OOPS in crash_shrink_memory and shows
"/sys/kernel/kexec_crash_size" as 0 when crash kernel memory is not
reserved.
Signed-off-by: Pavan Naregundi <pavan@linux.vnet.ibm.com>
Reviewed-by: WANG Cong <xiyou.wangcong@gmail.com>
Cc: Simon Horman <horms@verge.net.au>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/kexec.c

@@ -1089,8 +1089,9 @@ void crash_kexec(struct pt_regs *regs)
 
 size_t crash_get_memory_size(void)
 {
-	size_t size;
+	size_t size = 0;
 	mutex_lock(&kexec_mutex);
+	if (crashk_res.end != crashk_res.start)
 		size = crashk_res.end - crashk_res.start + 1;
 	mutex_unlock(&kexec_mutex);
 	return size;
@@ -1134,7 +1135,7 @@ int crash_shrink_memory(unsigned long new_size)
 
 	free_reserved_phys_range(end, crashk_res.end);
 
-	if (start == end)
+	if ((start == end) && (crashk_res.parent != NULL))
 		release_resource(&crashk_res);
 	crashk_res.end = end - 1;