bgpd: Trust the bgp_table_top_nolock for aggregates

bgpd: Trust the bgp_table_top_nolock for aggregates

The safi passed in to short-circuit the aggregate lookup
adds code complexity and little speed improvements for
the case where we actually may have aggregates configured!
Since bgp_table_top_nolock() actually tells us if there
are any aggregates installed and safely returns if there
is nothing to do, trust it.  As that we know for those
safi's were we don't want to have, we dissallow the
creation via the cli anyways.

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