If fastopen is used, poll must allow a first write that will trigger
the SYN+data

Similar to what is done in tcp_poll().
Acked-by: NPaolo Abeni <pabeni@redhat.com>
Reviewed-by: NMatthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: NBenjamin Hesmans <benjamin.hesmans@tessares.net>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>