Linux 6.12: support 3arg dequeue_signal() without task param

See torvalds/linux@a2b80ce87a87. It claims the task arg is always
current, and so it is with us, so this is a safe change to make. The
only spanner is that we also support the older pre-5.17 3-arg
dequeue_signal() which had different meaning, so we have to check the
types to get the right one.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #16582