bgpd: bgp_path_info_extra memory optimization

bgpd: bgp_path_info_extra memory optimization

Even if some of the attributes in bgp_path_info_extra are
not used, their memory is still allocated every time. It
cause a waste of memory.
This commit code deletes all unnecessary attributes and
changes the optional attributes to pointer storage. Memory
will only be allocated when they are actually used. After
optimization, extra info related memory is reduced by about
half(~400B -> ~200B).

Signed-off-by: Valerian_He <1826906282@qq.com>