dracut: fix typo in mount-zfs.sh.in

Format the zpool get command correctly. The -o option must
be followed by "all" or the requested field name.

Reviewed-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #13602