Fix zfs.spec.in to use @LICENSE@ variable

The zfs.spec.in file had the license field hard coded to specify the
CDDL. This was changed to use the @LICENSE@ variable, maintaining
consistency with the zfs-modules.spec.in file.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>