net: mpls: Make nla_get_via in af_mpls.c

nla_get_via is only used in af_mpls.c. Remove declaration from internal.h
and move up in af_mpls.c before first use. Code move only; no
functional change intended.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>