MIPS: fix mem=X@Y commandline processing

When a memory offset is specified through the commandline, add the
memory in range PHYS_OFFSET:Y as reserved memory area.
Otherwise the bootmem allocator is initialised with low page equal to
min_low_pfn = PHYS_OFFSET, and in free_all_bootmem will process pages
starting from min_low_pfn instead of PFN(Y).

Signed-off-by: Marcin Nowakowski <marcin.nowakowski@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/14613/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/setup.c

@@ -662,6 +662,10 @@ static int __init early_parse_mem(char *p)
 		start = memparse(p + 1, &p);
 
 	add_memory_region(start, size, BOOT_MEM_RAM);
+
+	if (start && start > PHYS_OFFSET)
+		add_memory_region(PHYS_OFFSET, start - PHYS_OFFSET,
+				BOOT_MEM_RESERVED);
 	return 0;
 }
 early_param("mem", early_parse_mem);