tipc: replace open-code bearer rcu_dereference access in bearer.c

tipc: replace open-code bearer rcu_dereference access in bearer.c

Replace these open-code bearer rcu_dereference access with bearer_get(),
like other places in bearer.c. While at it, also use tipc_net() instead
of net_generic(net, tipc_net_id) to get "tn" in bearer.c.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Larysa Zaremba <larysa.zaremba@intel.com>
Reviewed-by: Tung Nguyen <tung.q.nguyen@dektech.com.au>
Link: https://lore.kernel.org/r/1072588a8691f970bda950c7e2834d1f2983f58e.1685976044.git.lucien.xin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>