ofp: ensure that l2 in ofpmp_reserve()

Ensure that the buffer returned by ofpmp_reserve() has buf->l2 set
as this may be required by nxm_reg_load_to_nxast() when generating
the reply to an stats request

This problem was observed when dumping a large number of flows
with set_field actions using ovs-ofctl dump-flows.

Signed-off-by: Simon Horman <horms@verge.net.au>