tcp: fix a size_t < 0 comparison in tcp_read_sock

<used> should be of type int (not size_t) since recv_actor can return
negative values and it is also used in a < 0 comparison.

Signed-off-by: Octavian Purdila <opurdila@ixiacom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>