[sonic-linkmgrd][master] submodule update (#9420)

#### What I did 

[sonic-linkmgrd][master] submodule update

6c6151b Fix unstable unit tests (state change handler wasn't invoked) (#8)
2f7dc0a support code diff coverage (#5)
83f0002 Force mux state switch to standby if triggered from Cli (#6)

signed-off-by: Jing Zhang [email protected]