x2ap/sctp: increase buffer size

The 5G phone OPPO has a very big UE capability (> 2KB).

We need bigger buffers to transfer this, especially in x2 handover.

Also the sctp messages need a bigger buffer.

Note: in sctp, we didn't check for partial messages, it's now
      done. When detected, we exit. To be redone differently
      if needed. (Look for "if (!(flags & MSG_EOR))".)