LU-16524 sec: add fscrypt_admin rbac role

LU-16524 sec: add fscrypt_admin rbac role

The purpose of the new fscrypt_admin rbac role is to control admin
tasks related to fscrypt. When not set, it is forbidden to all users
including root to modify existing protectors or policies, or create
new ones. But it remains possible to lock and unlock encrypted
directories.

Internally, this is achieved by marking fscrypt metadata files and
directories, i.e. everything under ROOT/.fscrypt, with a special mdt
object flag LOHA_FSCRYPT_MD.
Upon request processing, the mdt layer returns -EPERM if the flag
LOHA_FSCRYPT_MD is found on an object that is the target of a modify
request.
The LUSTRE_IMMUTABLE_FL flag is also returned to clients for such
objects.

sanity-sec test_64f is added to exercise the new fscrypt_admin flag.

Lustre-change: https://review.whamcloud.com/50184
Lustre-commit: 22bef9b6c64ef394a2efb41ce1388be71300af0d

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I99956499133994444ccd88e33340067790a182ce
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/50339
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>