tcp: Remove unrequired operations in tcp_push()

Remove unrequired operations in tcp_push()

Changelog:
	Removed a temporary skb variable from tcp_push()

Signed-off-by: Krishna Kumar <krkumar2@in.ibm.com>
Acked-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>