tcp_get_fds API discards fds if there's more than 1 of these.

It's tricky to fix this without API changes in the generic case.

However, this API is only used by tests ATM, and tests know how
many fds they expect.

So let's not waste cycles trying to fix this properly:
simply assume at most 16 fds (tests use at most 8 now).
assert if some test tries to get more.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>