fs: dlm: fix typo in tlv prefix

This patch fixes a small typo in a unused struct field. It should named
be t_pad instead of o_pad. Came over this as I updated wireshark
dissector.

Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>