kmod: Handle UMH_WAIT_PROC from system unbound workqueue

The UMH_WAIT_PROC handler runs in its own thread for obsolete reasons.
We couldn't launch and then wait for the exec kernel thread completion
without blocking other usermodehelper queued jobs since khelper was
implemented as a singlthread ordered workqueue.

But now we replaced khelper with generic system unbound workqueues which
can handle concurrent blocking jobs.

So lets run it from the workqueue.

CHECK: I'm just worried about the signal handler that gets tweaked
and also the call to sys_wait() that might fiddle with internals. The
system workqueue must continue to work without surprise for other
works.

Cc: Rik van Riel <riel@redhat.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Christoph Lameter <cl@linux.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>