LU-14651 ldiskfs: add 5.11 kernel support

The Ubuntu 20.04.3 LTS moved to the 5.11 kernel. Support for this
kernel is a small step from the 5.10 kernel support. This patch
adds these small changes to support ldiskfs.

Test-Parameters: trivial
Change-Id: I3055736658b628fe79a6a9fc20ac01e7e1597630
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/47900
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>