Add spl-dkms dependency to zfs-dkms

Adding the 'spl-dkms = VERSION' dependency to the zfs-dkms
package ensures the spl will be installed before zfs.  This
cleanly handles the initial 'yum localinstall' case.

However, this does not address the dkms rebuilds caused by
a new kernel being installed.  For that we still rely on the
clunky  --with-spl-timeout=<timeout> configure option which
allows the zfs compilation to be briefly delayed while the
spl components are built.

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