[submodule] Update submodule sonic-linux-kernel to the latest HEAD au…

…tomatically (#15784)

#### Why I did it
src/sonic-linux-kernel
```
* 694b338 - (HEAD -> 202012, origin/202012) [202012] Add crypto/zstd.ko module via CONFIG_CRYPTO_ZSTD (#320) (4 weeks ago) [Samuel Angebault]
```