ANDROID: fix binder change in merge of 4.9.188

The 4.9.188 merge was missing the change to the
binder driver associated with the linux-4.9.y
commit 16903f1a ("coredump: fix race condition
between mmget_not_zero()/get_task_mm() and core dumping").
It was left out because the android-4.9 binder
driver has been significantly refactored compared
to linux-4.9.y.

This patch applies the missing change from that
patch to the binder driver.

Change-Id: I803e558cb629fedff04b14e23ac9f83e98628ede
Fixes: d4fff2d0 ("Merge 4.9.188 into android-4.9")
Signed-off-by: Todd Kjos <tkjos@google.com>