LU-13783 sec: support of native Ubuntu 20.04 HWE 5.8 kernel

LU-13783 sec: support of native Ubuntu 20.04 HWE 5.8 kernel

For Linux 5.5 kernel a patch to improve nokey names was landed
that removed several variables that Lustre's llite and mdt layer
were using. Rework the code to use other defines that exist.
Second change for Ubuntu is several backports to handle a few
variables changing across different kernel versions. One is the name
change of DCACHE_ENCRYPTED_NAME and the other being
is_chipertext_name. So the simpler approach Ubuntu took was to use
fscrypt_prepare_lookup() and fscrypt_is_nokey_name() to work
around these changes. Lastly the good news is for 5.12 the
stomping of ll_d_ops no longer happens and the special
revalidate_dentry fscrypto function is exported.

Change-Id: I7f70fe9abddf34798e2e01b35099c9a032d92b91
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-on: https://review.whamcloud.com/46238
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>