of/fdt: Fix the len check in early_init_dt_check_for_elfcorehdr()

The len value is in bytes, while `dt_root_addr_cells + dt_root_size_cells`
is in cells (4 bytes per cell). Comparing them directly is incorrect.

Use a helper function to simplify the code and address this issue.

Fixes: f7e7ce9 ("of: fdt: Add generic support for handling elf core headers property")
Fixes: e62aaea ("arm64: kdump: provide /proc/vmcore file")
Signed-off-by: Yuntao Wang <yuntao.wang@linux.dev>
Link: https://patch.msgid.link/20251115134753.179931-3-yuntao.wang@linux.dev
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>

Author: ytcoode

drivers/of/fdt.c

@@ -853,21 +853,15 @@ static void __init early_init_dt_check_for_initrd(unsigned long node)
  */
 static void __init early_init_dt_check_for_elfcorehdr(unsigned long node)
 {
-	const __be32 *prop;
-	int len;
-
 	if (!IS_ENABLED(CONFIG_CRASH_DUMP))
 		return;
 
 	pr_debug("Looking for elfcorehdr property... ");
 
-	prop = of_get_flat_dt_prop(node, "linux,elfcorehdr", &len);
-	if (!prop || (len < (dt_root_addr_cells + dt_root_size_cells)))
+	if (!of_flat_dt_get_addr_size(node, "linux,elfcorehdr",
+				      &elfcorehdr_addr, &elfcorehdr_size))
 		return;
 
-	elfcorehdr_addr = dt_mem_next_cell(dt_root_addr_cells, &prop);
-	elfcorehdr_size = dt_mem_next_cell(dt_root_size_cells, &prop);
-
 	pr_debug("elfcorehdr_start=0x%llx elfcorehdr_size=0x%llx\n",
 		 elfcorehdr_addr, elfcorehdr_size);
 }