x86/crash: Use resource_size()

Use resource_size() rather than a verbose computation on
the end and start fields.

The semantic patch that makes this change is as follows:
(http://coccinelle.lip6.fr/)

<smpl>
@@ struct resource ptr; @@
- (ptr.end - ptr.start + 1)
+ resource_size(&ptr)
</smpl>
Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/1577900990-8588-10-git-send-email-Julia.Lawall@inria.fr

arch/x86/kernel/crash.c

@@ -370,7 +370,7 @@ int crash_setup_memmap_entries(struct kimage *image, struct boot_params *params)
 	/* Add crashk_low_res region */
 	if (crashk_low_res.end) {
 		ei.addr = crashk_low_res.start;
-		ei.size = crashk_low_res.end - crashk_low_res.start + 1;
+		ei.size = resource_size(&crashk_low_res);
 		ei.type = E820_TYPE_RAM;
 		add_e820_entry(params, &ei);
 	}