LU-14776 ldiskfs: Add Ubuntu 20.04 HWE support

LU-14776 ldiskfs: Add Ubuntu 20.04 HWE support

Use the already landed ldiskfs support for Linux 5.8.0 to enable
support for the Ubuntu 20.04 HWE 5.8.0-53 kernel. Another change
that started with the 5.7 kernel is removal of the flag
EXT4_GET_BLOCKS_KEEP_SIZE. The code was no longer needed with the
removal of EXT4_EOFBLOCKS_FL which happened in 2012. e2fsprog
support for this flag has been removed since version 1.42.2.

Change-Id: I60db446bab50178a601e1c2c20e782435f9f50f2
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/44039
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>