mptcp: don't grow mptcp socket receive buffer when rcvbuf is locked

The mptcp rcvbuf size is adjusted according to the subflow rcvbuf size.
This should not be done if userspace did set a fixed value.

Fixes: 600911ff ("mptcp: add rmem queue accounting")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>