ANDROID: binder: fix proc->tsk check.

Commit c4ea41ba ("binder: use group leader instead of open thread")'
was incomplete and didn't update a check in binder_mmap(), causing all
mmap() calls into the binder driver to fail.

Signed-off-by: Martijn Coenen <maco@android.com>
Tested-by: John Stultz <john.stultz@linaro.org>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>