[PATCH] ibmveth fix failed addbuf

This patch fixes a bug that happens when the hypervisor can't add a
buffer.  The old code wrote IBM_VETH_INVALID_MAP into the free_map
array, so next time the index was used, a ibmveth_assert() caught it and
called BUG().  The patch writes the right value into the free_map array
so that the index can be reused.

Signed-off-by: Santiago Leon <santil@us.ibm.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>