bgpd: fix redistribute table command after bgp restarts

bgpd: fix redistribute table command after bgp restarts

When the BGP 'redistribute table' command is used for a given route
table, and BGP configuration is flushed and rebuilt, the redistribution
does not work.

Actually, when flushing the BGP configuration with the 'no router bgp'
command, the BGP redistribute entries related to the 'redistribute table'
entries are not flushed. Actually, at BGP deletion, the table number is
not given as parameter in bgp_redistribute_unset() function, and the
redistribution entry is not removed in zebra.
Fix this by adding some code to flush all the redistribute table
instances.

Fixes: 7c8ff89e9346 ("Multi-Instance OSPF  Summary")
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>