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 c7c1e3846bac1e4b8a8941f6a194812e28b0a519

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

arch/mips/mm/fault.c

@@ -171,6 +171,7 @@ out_of_memory:
 	 * We ran out of memory, call the OOM killer, and return the userspace
 	 * (which will retry the fault, or kill us if we got oom-killed).
 	 */
+	up_read(&mm->mmap_sem);
 	pagefault_out_of_memory();
 	return;