Use known license string for zlua

The Linux kernel MODULE_LICENSE macro only recognizes a handful of
license strings and "MIT" is not one of the them. Update the macro
to use "Dual MIT/GPL" which is recognized and what the kernel expects
MIT licensed modules to use.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11112
Closes #11113