networking: remove double socket unlock

sogeneric_send(), in one error case, unlocks the socket before doing
'goto release' which unlocks the socket again, resulting in an assertion
failure (unlocking an unlocked mutex) and crash.

This patch removes the extra unlock.

Fixes #248.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>