Fix directory detection in dkms.mkconf

Fix zfs-dkms installation on Debian-derived distributions by
aligning the directory detection logic to #13096.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: szubersk <szuberskidamian@gmail.com>
Closes #11449
Closes #13141