kexec: make a pair of map/unmap reserved pages in error path

For some arch, kexec shall map the reserved pages, then use them, when
we try to start the kdump service.

kexec may return directly, without unmaping the reserved pages, if it
fails during starting service. To fix it, we make a pair of map/unmap
reserved pages both in generic path and error path.

This patch only affects s390. Other architecturess don't implement the
interface of crash_unmap_reserved_pages and crash_map_reserved_pages.

It isn't a urgent patch. Kernel can work well without any risk,
although the reserved pages are not unmapped before returning in error
path.

Signed-off-by: Minfei Huang <[e-mail]>
Cc: Vivek Goyal <[e-mail]>
Cc: "Eric W. Biederman" <[e-mail]>
Cc: Xunlei Pang <[e-mail]>
Cc: Baoquan He <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/kexec.c

···
 						   segments, flags);
 		}
 		if (result)
-			goto out;
+			goto unmap_page;
 
 		if (flags & KEXEC_PRESERVE_CONTEXT)
 			image->preserve_context = 1;
 		result = machine_kexec_prepare(image);
 		if (result)
-			goto out;
+			goto unmap_page;
 
 		for (i = 0; i < nr_segments; i++) {
 			result = kimage_load_segment(image, &image->segment[i]);
 			if (result)
-				goto out;
+				goto unmap_page;
 		}
 		kimage_terminate(image);
+unmap_page:
 		if (flags & KEXEC_ON_CRASH)
 			crash_unmap_reserved_pages();
+		if (result)
+			goto out;
 	}
 	/* Install the new kernel, and  Uninstall the old */
 	image = xchg(dest_image, image);