LU-14577 ldiskfs: support Ubuntu 20.04 kernel 5.4.0-1007

LU-14577 ldiskfs: support Ubuntu 20.04 kernel 5.4.0-1007

While applying 5.4.0-66-ubuntu20.series ldiskfs patches
to kernel 5.4.0-1007, there is a conflict in
ext4_update_dx_flag() in ubuntu2004/ext4-pdirop.patch.
It turns out the ext4_update_dx_flag() codes in kernel
5.4.0-1007 are the same with those in kernel version
smaller than 5.4.0-66. So, 5.4.0-42-ubuntu20.series works.

This patch fixes lustre-build-ldiskfs.m4 to detect
5.4.0-42-ubuntu20.series for kernel 5.4.0-1007.

Test-Parameters: trivial

Change-Id: I3cd932b8ae2d7c7f4f900b8b18647a4252d100b2
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/43187
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>