MAINTAINERS: ipvs: move Wensong Zhang to CREDITS

Move Wensong Zhang to credits, we haven't heard from
him in years.

Subsystem IPVS
  Changes 83 / 226 (36%)
  Last activity: 2020-11-27
  Wensong Zhang <wensong@linux-vs.org>:
  Simon Horman <horms@verge.net.au>:
    Committer c24b75e0 2019-10-24 00:00:00 33
    Tags 7980d2ea 2020-10-12 00:00:00 76
  Julian Anastasov <ja@ssi.bg>:
    Author 7980d2ea 2020-10-12 00:00:00 26
    Tags 4bc3c8dc 2020-11-27 00:00:00 78
  Top reviewers:
    [6]: horms+renesas@verge.net.au
  INACTIVE MAINTAINER Wensong Zhang <wensong@linux-vs.org>

Signed-off-by: Jakub Kicinski <kuba@kernel.org>