LU-11838 ldiskfs: add rhel8 server support

LU-11838 ldiskfs: add rhel8 server support

This patch adds ldiskfs patch series for rhel8
kernel 4.18.0-32.el8.

Fix lustre-build-ldiskfs.m4, make
CONFIG_LDISKFS_FS_ENCRYPTION consistent with
kernel's config CONFIG_EXT4_FS_ENCRYPTION.
Otherwise ldiskfs won't build
on kernels with CONFIG_EXT4_FS_ENCRYPTION
disabled.

Note: this contains a small clean up for
ubuntu18/ext4-kill-dx-root.patch

Test-Parameters:trivial
Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Change-Id: Ib500bff2f6688405b912620c5217586c8420c6e1
Reviewed-on: https://review.whamcloud.com/34674
Reviewed-by: Shaun Tancheff <stancheff@cray.com>
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>