f2fs: Enlarge min_fsync_blocks to 20

In OPPO's kernel:
enlarge min_fsync_blocks to optimize performance
  - yanwu@TECH.Storage.FS.oF2FS, 2019/08/12

Huawei is also doing this in their production kernel.

If this optimization is good for them and shipped
with their devices, it should be good for us.

Signed-off-by: Jesse Chan <jc@linux.com>
Signed-off-by: DennySPb <dennyspb@gmail.com>
Signed-off-by: Chenyang Zhong <zhongcy95@gmail.com>
Signed-off-by: Marco Zanin <mrczn.bb@gmail.com>
Signed-off-by: improbir <ghosh_probir@yahoo.com>