um: Reap winch thread if it fails

When the winch thread runs into an error condition, it would exit(1) and
never be reaped until shutdown time. Change this to write a command byte
which causes the driver to kill it, therefore reaping the child.

Signed-off-by: Benjamin Berg <benjamin@sipsolutions.net>
Signed-off-by: Richard Weinberger <richard@nod.at>