tcp: Kill extraneous SPLICE_F_NONBLOCK checks.

In splice TCP receive, the SPLICE_F_NONBLOCK flag is used
to compute the "timeo" value.  So checking it again inside
of the main receive loop to trigger -EAGAIN processing is
entirely unnecessary.

Noticed by Jarek P. and Lennert Buytenhek.

Signed-off-by: David S. Miller <davem@davemloft.net>