contrib: link zpool to zfs in bash-completion (#16376)

Currently user won't have completion of zpool command until they
trigger completion of zfs first. This patch adds a link to zfs,
thus user can use both to initialize the completion.

Fixes: #16320

Signed-off-by: Shengqi Chen <harry-chen@outlook.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Rob Norris <rob.norris@klarasystems.com>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>