tipc: force a dst refcount before doing decryption

As it says in commit 3bc07321 ("xfrm: Force a dst refcount before
entering the xfrm type handlers"):

"Crypto requests might return asynchronous. In this case we leave the
 rcu protected region, so force a refcount on the skb's destination
 entry before we enter the xfrm type input/output handlers."

On TIPC decryption path it has the same problem, and skb_dst_force()
should be called before doing decryption to avoid a possible crash.

Shuang reported this issue when this warning is triggered:

  [] WARNING: include/net/dst.h:337 tipc_sk_rcv+0x1055/0x1ea0 [tipc]
  [] Kdump: loaded Tainted: G W --------- - - 4.18.0-496.el8.x86_64+debug
  [] Workqueue: crypto cryptd_queue_worker
  [] RIP: 0010:tipc_sk_rcv+0x1055/0x1ea0 [tipc]
  [] Call Trace:
  [] tipc_sk_mcast_rcv+0x548/0xea0 [tipc]
  [] tipc_rcv+0xcf5/0x1060 [tipc]
  [] tipc_aead_decrypt_done+0x215/0x2e0 [tipc]
  [] cryptd_aead_crypt+0xdb/0x190
  [] cryptd_queue_worker+0xed/0x190
  [] process_one_work+0x93d/0x17e0

Fixes: fc1b6d6d

 ("tipc: introduce TIPC encryption & authentication")
Reported-by: Shuang Li <shuali@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Link: https://lore.kernel.org/r/fbe3195fad6997a4eec62d9bf076b2ad03ac336b.1718476040.git.lucien.xin@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>