net: Make mpls_entry_encode() available for generic users

Move mpls_entry_encode() from net/mpls/internal.h to include/net/mpls.h
and make it available for other users. Specifically, hardware driver that
offload MPLS can benefit from that.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Suggested-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>