MIPS: bootmem: Fix mapstart calculation for contiguous maps

MIPS: bootmem: Fix mapstart calculation for contiguous maps

commit 88d3426942d748b90b051b7ef2d5d765f5f3054c upstream.

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: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/11086/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>