lxc_fuse: Prefer fuse_file_info::direct_io over mount option

lxc_fuse: Prefer fuse_file_info::direct_io over mount option

When mounting a FUSE it is possible to bypass kernel cache by
specifying -odirect_io mount option. This is what we currently
do. However, FUSEv3 has a different approach - the open callback
(lxcProcOpen() in our case) can set direct_io member of
fuse_file_info struct. This results in the same behaviour, but
also works with both FUSEv1 and FUSEv3. The latter does not have
the mount option and uses per file approach.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>