mm: skip swap readahead when process is exiting

It's possible for a user fault to be triggered during task exit that
results in swap readahead, which is not useful. Skip swap readahead
if the current process is exiting.

Change-Id: I5fad20ebdcc616af732254705726d395eb118cbe
Signed-off-by: Tim Murray <timmurray@google.com>