fix(mount): do not truncate shares not zfs mount

When running zfs share -a resetting the exports.d/zfs.exports makes
sense the get a clean state.
Truncating was also called with zfs mount which would not populate the
file again.
Add test to verify shares persist after mount -a.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Stefan Lendl <s.lendl@proxmox.com>
Closes #15607
Closes #15660