usermodehelper: ____call_usermodehelper() doesn't need do_exit()

usermodehelper: ____call_usermodehelper() doesn't need do_exit()

Minor cleanup.  ____call_usermodehelper() can simply return, no need to
call do_exit() explicitely.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Tejun Heo <tj@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>