mm: Export do_munmap

The 0-day build bot reports the following build error, seen if SDCARD_FS
is built as module.

ERROR: "do_munmap" undefined!

Fixes: 84a1b7d3 ("Included sdcardfs source code for kernel 3.0")
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Guenter Roeck <groeck@chromium.org>