net: openvswitch: do not update max_headroom if new headroom is equal to old headroom

When a vport is deleted, the maximum headroom size would be changed.
If the vport which has the largest headroom is deleted,
the new max_headroom would be set.
But, if the new headroom size is equal to the old headroom size,
updating routine is unnecessary.

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Tested-by: Greg Rose <gvrose8192@gmail.com>
Reviewed-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>