Fix zfs.spec.in defaults

Commit 2ee4e7da accidentally introduced two issues which only occur
when rebuilding the ZFS source rpm outside the ZFS build system.

1) The _dracutdir, _udevdir, and _udevruledir macros must be checked
   using the 'undefined' keyword.  This was just overlooked in the
   patch review and does not cause a failure when using 'make pkg'
   because the values are provided by the make target.

2) The default _udevruledir path included a typo.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2310
Tag zfs-0.6.3

META file and release log updated.

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