do not restart most services upon upgrade

do not restart most services upon upgrade

zfs-zed is the only classical daemon, which benefits from a restart,
so restart only zfs-zed.

Noticed during the transition from ZFS 0.8.x to 2.0.0 zvol_wait, which
is called by zfs-volume-wait.service was changed to also output the
redacted_snap property of the datasets. This property does not exist
if the kernelmodule is still 0.8.x, causing an error to be printed to
the journal.

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>