Registering a winch IRQ is racy, an interrupt may occur before the winch is
added to the winch_handlers list.

If that happens, register_winch_irq() adds to that list a winch that is
scheduled to be (or has already been) freed, causing a panic later in
winch_cleanup().

Avoid the race by adding the winch to the winch_handlers list before
registering the IRQ, and rolling back if um_request_irq() fails.

Fixes: 42a359e3 ("uml: SIGIO support cleanup")
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Richard Weinberger <richard@nod.at>