LU-13740 ldiskfs: add Ubuntu 20.04 LTS support

LU-13740 ldiskfs: add Ubuntu 20.04 LTS support

The latest Ubuntu LTS is based on a 5.4 Linux kernel. We can use
the previous work of generic 5.4 ldiskfs support with a small
change to the ext-pdirop.patch to make ldiskfs available to
this latest Ubuntu LTS.

Change-Id: I6d1106c3da984b508ccc77364e13adc692bddb60
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/39779
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>