linux: log a scary warning when used with an experimental kernel

Since the person using the kernel may not be the person who built it,
show a warning at module load too, in case they aren't aware that it
might be weird.

Reviewed-by: Robert Evans <evansr@google.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Closes #15986