iscsi: Fix up ipv6 in brackets, iterate over all possible needed iscsi mounts

for_each_host_dev_and_slaves only iterates over the rootfs, this one is
successful and then it exits.

This does not work if the --mount option is passed on a iscsi target.
Now really iterate over all host_devs via:
for_each_host_dev_and_slaves_all

and set needed iscsi kernel commandline accordingly.
Now also the parent block device, e.g. /dev/sda and /dev/sda4 are iterated
which results in duplicated iscsi parameters.
Avoid this by piping the final iscsi cmdline function through | sort |uniq

Signed-off-by: Thomas Renninger <trenn@suse.de>