bgpd: Make strip extcommunity handle multiple extcommunities

bgpd: Make strip extcommunity handle multiple extcommunities

Extended communities like the BGP Route Target can be present multiple
times in a route's path attribute. Ensure that the strip function for a
particular extended community (type and subtype) handles this and
strips all occurrences.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by:   Don Slice <dslice@cumulusnetworks.com>