um: fix sparse warnings in signal code

sparse reports that various places were missing the __user tag in casts.
In addition, one location was using 0 instead of NULL.

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20241031142017.430420-2-benjamin@sipsolutions.net


Signed-off-by: Johannes Berg <johannes.berg@intel.com>