net: pppolac/pppopns: Replace msg.msg_iov with iov_iter_kvec()

Commit 1af89c1e ("Hack: net: PPPoPNS and PPPoLAC build fixes for 4.1")
fixed the build for PPPoPNS and PPPoLAC by re-introducing a field in
struct msghdr which was removed upstream. Re-introducing the field doesn't
get it used, so it is quite likely that the code never worked. Fix it up for
good.

Fixes: 1af89c1e ("Hack: net: PPPoPNS and PPPoLAC build fixes for 4.1")
Signed-off-by: Guenter Roeck <groeck@chromium.org>