vfs_ceph_new: ref cephmount_cached entry in handle->data

vfs_ceph_new: ref cephmount_cached entry in handle->data

Allow direct access to ceph-mount cached-entry via 'handle->data'
private pointer. Required in order to allow more complex cached-state
with each cephfs mount. Users should now use the local-helper function
'cmount_of' to access the underlying ceph_mount_info.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15686

Signed-off-by: Shachar Sharon <ssharon@redhat.com>
Reviewed-by: Guenther Deschner <gd@samba.org>
Reviewed-by: Anoop C S <anoopcs@samba.org>