LU-15848 ldiskfs: escape encrypted file names

LU-15848 ldiskfs: escape encrypted file names

When a Lustre MDT is mounted as ldiskfs, the names of the encrypted
files have to be escaped to avoid breaking the shell.
On CentOS 7, the LDISKFS_ENCRYPT_FL flag does not exist. So we add it,
and when the target is mounted as ldiskfs (LDISKFS_MOUNT_DIRDATA flag
not present) we critical-encode encrypted file names before
presentation. And conversely, we critical-decode names upon lookup.
On CentOS 8, the LDISKFS_ENCRYPT_FL flag exists. The fscrypt functions
from kernel 4.18 are also wired up, but they all refer to -EOPNOTSUPP
or equivalent, so they cannot be used to present usable names. So when
the target is mounted as ldiskfs (LDISKFS_MOUNT_DIRDATA flag not
present) we proceed to critical-encoding of encrypted file names
before presentation, and to critical-decoding upon lookup.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Iaca467eaa233be8142356efa822962953754c2ce
Reviewed-on: https://review.whamcloud.com/47309
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>