Fix Dracut scripts to allow for blanks in pool and dataset names

The ability to use blanks is documented in zpool(8) and implemented
in module/zcommon/zfs_namecheck.c:valid_char().

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3083