kernel: mptcp: prevent BPF accessing lowat from a subflow socket.

In the Linux kernel, the following vulnerability has been resolved: mptcp: prevent BPF accessing lowat from a subflow socket. Alexei reported the following splat: WARNING: CPU: 32 PID: 3276 at net/mptcp/subflow.c:1430 subflowdataready+0x147/0x1c0 Modules linked in: dummy bpftestmodO last unloaded...