IB/rxe: Unblock loopback by moving skb_out increment

skb_out is decremented in rxe_skb_tx_dtor(), which is not called in the
loopback() path. Move the increment to the send() path rather than
rxe_xmit_packet().

Signed-off-by: Andrew Boyer <andrew.boyer@dell.com>
Acked-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>