Add dkms_version conditional

By adding a dkms_version conditional it's now possible to specify
an exact version of dkms.   This is used by the Fedora and EPEL
yum repositories to ensure the patched version of dkms provided
by the repository is installed.  The patched version of dkms
ensures that the spl modules are built before the zfs modules.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1466