tipc: increment the tmp aead refcnt before attaching it

Li Shuang found a NULL pointer dereference crash in her testing:

  [] BUG: unable to handle kernel NULL pointer dereference at 0000000000000020
  [] RIP: 0010:tipc_crypto_rcv_complete+0xc8/0x7e0 [tipc]
  [] Call Trace:
  []  <IRQ>
  []  tipc_crypto_rcv+0x2d9/0x8f0 [tipc]
  []  tipc_rcv+0x2fc/0x1120 [tipc]
  []  tipc_udp_recv+0xc6/0x1e0 [tipc]
  []  udpv6_queue_rcv_one_skb+0x16a/0x460
  []  udp6_unicast_rcv_skb.isra.35+0x41/0xa0
  []  ip6_protocol_deliver_rcu+0x23b/0x4c0
  []  ip6_input+0x3d/0xb0
  []  ipv6_rcv+0x395/0x510
  []  __netif_receive_skb_core+0x5fc/0xc40

This is caused by NULL returned by tipc_aead_get(), and then crashed when
dereferencing it later in tipc_crypto_rcv_complete(). This might happen
when tipc_crypto_rcv_complete() is called by two threads at the same time:
the tmp attached by tipc_crypto_key_attach() in one thread may be released
by the one attached by that in the other thread.

This patch is to fix it by incrementing the tmp's refcnt before attaching
it instead of calling tipc_aead_get() after attaching it.

Fixes: fc1b6d6d

 ("tipc: introduce TIPC encryption & authentication")
Reported-by: Li Shuang <shuali@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>