MAINTAINERS: vrf: move Shrijeet to CREDITS

Shrijeet has moved on from VRF-related work.

Subsystem VRF
  Changes 30 / 120 (25%)
  Last activity: 2020-12-09
  David Ahern <dsahern@kernel.org>:
    Author 1b6687e3 2020-07-23 00:00:00 1
    Tags 9125abe7 2020-12-09 00:00:00 4
  Shrijeet Mukherjee <shrijeet@gmail.com>:
  Top reviewers:
    [13]: dsahern@gmail.com
    [4]: dsa@cumulusnetworks.com
  INACTIVE MAINTAINER Shrijeet Mukherjee <shrijeet@gmail.com>

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