VM: add "vm_munmap()" helper function

Like the vm_brk() function, this is the same as "do_munmap()", except it
does the VM locking for the caller.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/fs/aio.c b/fs/aio.c
index da88760..99bd790 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -92,11 +92,8 @@
 	for (i=0; i<info->nr_pages; i++)
 		put_page(info->ring_pages[i]);
 
-	if (info->mmap_size) {
-		down_write(&ctx->mm->mmap_sem);
-		do_munmap(ctx->mm, info->mmap_base, info->mmap_size);
-		up_write(&ctx->mm->mmap_sem);
-	}
+	if (info->mmap_size)
+		vm_munmap(ctx->mm, info->mmap_base, info->mmap_size);
 
 	if (info->ring_pages && info->ring_pages != info->internal_pages)
 		kfree(info->ring_pages);