mptcp: Correctly check for waiting application before updating sndbuf

In tcp_check_space, we circumvent the check for sk_socket and
SOCK_NOSPACE in order to force the subflow's call to sk_write_space.

However, tcp_new_space does a call to tcp_should_expand_sndbuf before
calling sk_write_space. So, in mptcp_should_expand_sndbuf, we can do the
correct check again. That way we avoid to needlessly increase the
send-buffer.

Reported-by: Mikel Jimenez <mikeljim@fb.com>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>