ceph: allow idmapped mounts

Now that we converted cephfs internally to account for idmapped mounts
allow the creation of idmapped mounts on by setting the FS_ALLOW_IDMAP
flag.

Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>