kexec: improve the readability of crash_shrink_memory()

The major adjustments are:
1. end = start + new_size.
   The 'end' here is not an accurate representation, because it is not the
   new end of crashk_res, but the start of ram_res, difference 1. So
   eliminate it and replace it with ram_res->start.
2. Use 'ram_res->start' and 'ram_res->end' as arguments to
   crash_free_reserved_phys_range() to indicate that the memory covered by
   'ram_res' is released from the crashk. And keep it close to
   insert_resource().
3. Replace 'if (start == end)' with 'if (!new_size)', clear indication that
   all crashk memory will be shrunken.

No functional change.

Link: https://lkml.kernel.org/r/20230527123439.772-5-thunder.leizhen@huawei.comSigned-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Acked-by: Baoquan He <bhe@redhat.com>
Cc: Cong Wang <amwang@redhat.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

kernel/kexec_core.c

@@ -1108,7 +1108,6 @@ ssize_t crash_get_memory_size(void)
 int crash_shrink_memory(unsigned long new_size)
 {
 	int ret = 0;
-	unsigned long start, end;
 	unsigned long old_size;
 	struct resource *ram_res;
 
@@ -1119,9 +1118,7 @@ int crash_shrink_memory(unsigned long new_size)
 		ret = -ENOENT;
 		goto unlock;
 	}
-	start = crashk_res.start;
-	end = crashk_res.end;
-	old_size = (end == 0) ? 0 : end - start + 1;
+	old_size = !crashk_res.end ? 0 : resource_size(&crashk_res);
 	new_size = roundup(new_size, KEXEC_CRASH_MEM_ALIGN);
 	if (new_size >= old_size) {
 		ret = (new_size == old_size) ? 0 : -EINVAL;
@@ -1134,22 +1131,20 @@ int crash_shrink_memory(unsigned long new_size)
 		goto unlock;
 	}
 
-	end = start + new_size;
-	crash_free_reserved_phys_range(end, crashk_res.end);
-
-	ram_res->start = end;
+	ram_res->start = crashk_res.start + new_size;
 	ram_res->end = crashk_res.end;
 	ram_res->flags = IORESOURCE_BUSY | IORESOURCE_SYSTEM_RAM;
 	ram_res->name = "System RAM";
 
-	if (start == end) {
+	if (!new_size) {
 		release_resource(&crashk_res);
 		crashk_res.start = 0;
 		crashk_res.end = 0;
 	} else {
-		crashk_res.end = end - 1;
+		crashk_res.end = ram_res->start - 1;
 	}
 
+	crash_free_reserved_phys_range(ram_res->start, ram_res->end);
 	insert_resource(&iomem_resource, ram_res);
 
 unlock: