[202012] Add crypto/zstd.ko module via CONFIG_CRYPTO_ZSTD (#320)

The feature `docker_inram` gets the maximum compression ratio when using
the `zstd` algorithm.

On more recent releases the crypto/zstd.ko module is built by default
but it's not on sonic-linux-kernel 4.19 hence this change.

I ran some benchmark applied to dockerfs.tar.gz on all compression algorithms available for zram.

```
Algo     Real Size  Comp Size Mem Size  Xtract Xtract+Sync Read  CRatio MRatio
lzo      1633783808 764522544 798195712 28.39  29.31       83.34 3.14   3.05
lzo-rle  1633787904 769651487 803340288 27.88  28.95       83.58 3.12   3.03
zstd     1634697216 573782415 602689536 26.28  66.27       91.68 3.85   3.71
lz4      1633783808 801108972 835792896 25.47  26.39       83.86 3.04   2.95
lz4hc    1634447360 738901758 773656576 39.49  71.14       82.53 3.21   3.11
```

Author: Staphylo

patch/kconfig-inclusions

@@ -2,6 +2,8 @@
 CONFIG_LOG_BUF_SHIFT=20
 
 [amd64]
+# For docker in RAM
+CONFIG_CRYPTO_ZSTD=m
 # For Arista
 CONFIG_I2C_MUX_PCA9541=m
 CONFIG_SENSORS_MAX6697=m