bgpd: set BGP_ATTR_MP_[UN]REACH_NLRI (fixes 1a211cb)

bgpd: set BGP_ATTR_MP_[UN]REACH_NLRI (fixes 1a211cb)

Unfortunately, the attribute present bits for MP_REACH and MP_UNREACH
which 1a211cb ("bgpd: one more fix"...) tests for are never set in their
corresponding attribute parsing functions.

Reported-by: Martin Winter <mwinter@netdef.org>
Fixes: 1a211cb "bgpd: one more fix for tightening of check for missing well-known attributes"
Cc: Paul Jakma <paul@opensourcerouting.org>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
(cherry picked from commit daefeb8755e194dd19a5f1910bc78d13c8147efb)