zfs: don't generate/update cachefile on pool import

zfs: don't generate/update cachefile on pool import

during storage activation.

for pools that don't get imported at boot (e.g. because their vdevs are
not available when zfs-import-*.service runs) it is fatal to include
them in the cachefile, for those that do get imported at boot this code
should never run anyway as they are already imported.

in any case, a fallback to import without cachefile is the safe variant.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>