MIPS: Drop mmap_sem in pagefault oom path

Fix the pagefault oom path which does not drop mm->mmap_sem.
This was introduced by commit c7c1e384

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>