contrib/dracut: zfs-lib: export_all: replace with inline zpool export -a

07a3312f170ac56cb480b0df9fdf4c83f116b59b, which introduced this in
October of 2014, didn't have zpool export -a available; we do

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13093