iscsi_ibft: Add prefix-len attr and display netmask

The iBFT table only specifies a prefix length, not a netmask.
And the netmask is pretty much pointless for IPv6.
So introduce a new attribute 'prefix-len'.

Some older user-space code might rely on the netmask attribute
being present, so we should always display it.

Changes from v1:
 - Combined two patches into one

Changes from v2:
 - Cleaned up/corrected wording for patch description

Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Lee Duncan <lduncan@suse.com>
Reviewed-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: Konrad Rzeszutek Wilk <konrad@kernel.org>