lib: refactor ll_proto functions

Move core logic of ll_proto_n2a and ll_proto_a2n
to utils.c and make it more generic by allowing to
pass table of protocols as argument (proto_tb).
Introduce struct proto with protocol ID and name to
allow this. This wil allow to use those functions by
other use cases.

Signed-off-by: Wojciech Drewek <wojciech.drewek@intel.com>
Acked-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David Ahern <dsahern@kernel.org>