[201811][utilities] advance utilities submodule head (#4490)

Submodule src/sonic-utilities d7e8f84cf..8c21fc151:
  > [utility] Filter FDB entries (#890)
  > Fix the warm-reboot script to support FRR based warm-reboot (#842)

Signed-off-by: Ying Xie <ying.xie@microsoft.com>