sctp: change to hold/put transport for proto_unreach_timer

sctp: change to hold/put transport for proto_unreach_timer

A call trace was found in Hangbin's Codenomicon testing with debug kernel:

  [ 2615.981988] ODEBUG: free active (active state 0) object type: timer_list hint: sctp_generate_proto_unreach_event+0x0/0x3a0 [sctp]
  [ 2615.995050] WARNING: CPU: 17 PID: 0 at lib/debugobjects.c:328 debug_print_object+0x199/0x2b0
  [ 2616.095934] RIP: 0010:debug_print_object+0x199/0x2b0
  [ 2616.191533] Call Trace:
  [ 2616.194265]  <IRQ>
  [ 2616.202068]  debug_check_no_obj_freed+0x25e/0x3f0
  [ 2616.207336]  slab_free_freelist_hook+0xeb/0x140
  [ 2616.220971]  kfree+0xd6/0x2c0
  [ 2616.224293]  rcu_do_batch+0x3bd/0xc70
  [ 2616.243096]  rcu_core+0x8b9/0xd00
  [ 2616.256065]  __do_softirq+0x23d/0xacd
  [ 2616.260166]  irq_exit+0x236/0x2a0
  [ 2616.263879]  smp_apic_timer_interrupt+0x18d/0x620
  [ 2616.269138]  apic_timer_interrupt+0xf/0x20
  [ 2616.273711]  </IRQ>

This is because it holds asoc when transport->proto_unreach_timer starts
and puts asoc when the timer stops, and without holding transport the
transport could be freed when the timer is still running.

So fix it by holding/putting transport instead for proto_unreach_timer
in transport, just like other timers in transport.

v1->v2:
  - Also use sctp_transport_put() for the "out_unlock:" path in
    sctp_generate_proto_unreach_event(), as Marcelo noticed.

Fixes: 50b5d6ad6382 ("sctp: Fix a race between ICMP protocol unreachable and connect()")
Reported-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Link: https://lore.kernel.org/r/102788809b554958b13b95d33440f5448113b8d6.1605331373.git.lucien.xin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>