tunnel: Zero keys if marked as not present.

If a key used to find a port then we mark it as being not present
(previously this was accomplished by setting it to zero).  However,
we also still pass the key to userspace which could lead to an
inconsistent state.  This also zeros out the key as before in order
to make our reported results consistent.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Kyle Mestery <kmestery@cisco.com>