net: bpf: Use the protocol's set_rcvlowat behavior if there is one

The commit d1361840 ("tcp: fix SO_RCVLOWAT and RCVBUF autotuning")
add one new (struct proto_ops)->set_rcvlowat method so that a protocol
can override the default setsockopt(SO_RCVLOWAT) behavior.

The prior bpf codes don't check and invoke the protos's set_rcvlowat,
now correct it.

Signed-off-by: Gao Feng <gfree.wind@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>