Move 90-zfs udev rule from dracut to udev/rules.d

This rule does not need to be dracut specific.  Automatically loading
the zfs module stack when a zfs device is detected is usually desirable.
My only concern is that this might cause trouble for large pools where
we don't want to automatically import the pool until all the disks are
available.  However, we'll cross that bridge when we come to it.

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