fix(iscsi): prefix syntax for static iBFT IPv6 addresses

fix(iscsi): prefix syntax for static iBFT IPv6 addresses

ibft_to_cmdline() formats a static IPv6 address like this
(without peer, gateway, hostname):

  ip="[fd09:9a46:b5c1:1fe:1::10/64]:::::ibft0:none"

This means that the "mask" field (4th) is left blank. When this is
parsed later by parse-ip-opts.sh, it bails out with the error
message "Sorry, automatic calculation of netmask is not yet supported".

parse-ip-opts.sh rather expects the prefix in the 4th field:

  ip="[fd09:9a46:b5c1:1fe:1::10]:::64::ibft0:none"

This syntax will be correctly transformed to the command

  ip addr add fd09:9a46:b5c1:1fe::10/64 dev ibft0

This patch fixes the formatting of the "ip=" line in ibft_to_cmdline().