Plug memory leak in zfsdev_state.

On kernel module unload, free all zfsdev state structures, except for
zfsdev_state_listhead, which is statically allocated.

Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Pawel Jakub Dawidek <pawel@dawidek.net>
Closes #14824