bgpd: Convert bgp_packet_mpattr_prefix_size to use an enum

bgpd: Convert bgp_packet_mpattr_prefix_size to use an enum

The function bgp_packet_mpattr_prefix_size had an if/else
body that allowed people to add encoding types to bgpd
such that we could build the wrong size packets.  This
was exposed recently in commit:
0a9705a1e07c1d8176fd21f8f1bde2a9a155331b

Where it was discovered flowspec was causing bgp update
messages to exceed the maximum size and the peer to
drop the connection.  Let's be proscriptive about this
and hopefully make it so that things don't work when
someone adds a new safi to the system ( and they'll have
to update this function ).

Signed-off-by: Donald Sharp <sharpd@nvidia.com>