[SCSI] iscsi: fix setting of pid from netlink skb

NETLINK_CREDS's pid now returns 0, so I guess we are supposed to
be using NETLINK_CB. This changed while the patch to export the
pid was getting merged upstream, so it was not noticed until both
the network and iscsi changes were in the same tree.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Cc: stable@kernel.org
Signed-off-by: James Bottomley <JBottomley@Parallels.com>