ceph: add fscrypt ioctls

ceph: add fscrypt ioctls

We gate most of the ioctls on MDS feature support. The exception is the
key removal and status functions that we still want to work if the MDS's
were to (inexplicably) lose the feature.

For the set_policy ioctl, we take Fs caps to ensure that nothing can
create files in the directory while the ioctl is running. That should
be enough to ensure that the "empty_dir" check is reliable.

Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>