contrib/initramfs: add missing conf.d/zfs

When upgrading from the distribution-provided zfs-initramfs package on
root-on-zfs Ubuntu and Debian the system may fail to boot: this change
adds the missing initramfs configuration file.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #7158