tipc: fix RCU dereference race in tipc_aead_users_dec()

tipc_aead_users_dec() calls rcu_dereference(aead) twice: once to store
in 'tmp' for the NULL check, and again inside the atomic_add_unless()
call.

Use the already-dereferenced 'tmp' pointer consistently, matching the
correct pattern used in tipc_aead_users_inc() and tipc_aead_users_set().

Fixes: fc1b6d6de220 ("tipc: introduce TIPC encryption & authentication")
Cc: stable@vger.kernel.org
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Daniel Hodges <hodgesd@meta.com>
Link: https://patch.msgid.link/20260203145621.17399-1-git@danielhodges.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>