bird2: Symbolic constants for bgp_origin don't work

[Luis Ressel]

Hello,

According to the docs, the bgp_origin route attribute can have the
values ORIGIN_IGP, ORIGIN_EGP and ORIGIN_INCOMPLETE. However, filter
conditions such as "bgp_origin = ORIGIN_IGP" don't appear to match
anything. (They don't throw an error either, so bird apparently
recognizes the constants.)

"bgp_origin = 0" (or 1, 2) works, though.

Cheers,
Luis