mptcp: Use del_init_rcu to reinitialize the pointers

Apparently strange things might happen otherwise...

Fixes: Bug #27 (https://github.com/multipath-tcp/mptcp/issues/27)
Reported-by: chenjianqing <https://github.com/cjqing0925>
Tested-by: chenjianqing <https://github.com/cjqing0925>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>