bgpd: Fix for BGP core when connected routes are redistributed

bgpd: Fix for BGP core when connected routes are redistributed
& GR is enabled.

When GR with deferral is enabled and connected routes are
distributed then in one race condition route node gets added
in to both deferred queue and work queue. If deferred queue
gets processed first then it ends up delete only flag while
leaving the entry in the work queue as it is. When a new update
comes for the same route node next time from peer then it hits
assert. Assert check is added to ensure we don’t add to work queue
again while it is already present.
So, check before adding in to deferred queue if it is already present
in work queue and bail if so.

Signed-off-by: Biswajit Sadhu <sadhub@vmware.com>