MIPS: bootmem: Fix mapstart calculation for contiguous maps

Commit a6335fa1 fixed the case with gap between initrd and next usable PFN zone,
but broken the case when initrd is combined with usable memory into one region
(in add_memory_region()). Restore the fixup initially brought in by f9a7febd.

---- error message ----
Unpacking initramfs...
Initramfs unpacking failed: junk in compressed archive
BUG: Bad page state in process swapper  pfn:00261
page:81004c20 count:0 mapcount:-127 mapping:  (null) index:0x2
flags: 0x0()
page dumped because: nonzero mapcount
CPU: 0 PID: 1 Comm: swapper Not tainted 4.2.0+ #1782
-----------------------

Signed-off-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Reported-by: Tony Wu <tung7970@gmail.com>
Tested-by: Tony Wu <tung7970@gmail.com>
Cc: David Daney <david.daney@cavium.com>
Cc: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Cc: Huacai Chen <chenhc@lemote.com>
Cc: Joe Perches <joe@perches.com>
Cc: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: stable@vger.kernel.org
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/11086/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/setup.c

@@ -338,7 +338,7 @@ static void __init bootmem_init(void)
 		if (end <= reserved_end)
 			continue;
 #ifdef CONFIG_BLK_DEV_INITRD
-		/* mapstart should be after initrd_end */
+		/* Skip zones before initrd and initrd itself */
 		if (initrd_end && end <= (unsigned long)PFN_UP(__pa(initrd_end)))
 			continue;
 #endif
@@ -371,6 +371,14 @@ static void __init bootmem_init(void)
 		max_low_pfn = PFN_DOWN(HIGHMEM_START);
 	}
 
+#ifdef CONFIG_BLK_DEV_INITRD
+	/*
+	 * mapstart should be after initrd_end
+	 */
+	if (initrd_end)
+		mapstart = max(mapstart, (unsigned long)PFN_UP(__pa(initrd_end)));
+#endif
+
 	/*
 	 * Initialize the boot-time allocator with low memory only.
 	 */