bgpd: Move extra free code and fix a bug.

bgpd: Move extra free code and fix a bug.

The bgp_info_extra_free code was the correct place to free
up data associated with the bgp_info pointer when we are
deleting the bgp_info node.

Additionally, if we have a parent pointer, we may not have a net
pointer.  So make sure we do.

Finally clean up the bgp_info_extra_free code so it is a bit
easier to read.  Use variables instead of multiple level
of casting.

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