MIPS: Avoid future duplicate elf core header reservation

Prepare for early_init_fdt_scan_reserved_mem() reserving the memory
occupied by an elf core header described in the device tree.
As arch_mem_init() calls early_init_fdt_scan_reserved_mem() before
mips_reserve_vmcore(), the latter needs to check if the memory has
already been reserved before.

Note that mips_reserve_vmcore() cannot just be removed, as not all MIPS
systems use DT.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>

arch/mips/kernel/setup.c

@@ -429,7 +429,8 @@ static void __init mips_reserve_vmcore(void)
 	pr_info("Reserving %ldKB of memory at %ldKB for kdump\n",
 		(unsigned long)elfcorehdr_size >> 10, (unsigned long)elfcorehdr_addr >> 10);
 
-	memblock_reserve(elfcorehdr_addr, elfcorehdr_size);
+	if (!memblock_is_region_reserved(elfcorehdr_addr, elfcorehdr_size)
+		memblock_reserve(elfcorehdr_addr, elfcorehdr_size);
 #endif
 }