[submodule]: update sonic-linux-kernel (#4856)

* c60b1f4 2020-06-26 | e1000: Do not perform reset in reset_task if we are already down (#148) (HEAD -> master, origin/master, origin/HEAD) [lguohan]
* c6aeedd 2020-06-25 | Updated NAT kernel patch for 4.19 buster (#147) [Akhilesh Samineni]

Signed-off-by: Guohan Lu <lguohan@gmail.com>