tipc: remove bearer_lock from tipc_bearer struct

After the earlier commits ("tipc: remove 'links' list from
tipc_bearer struct") and ("tipc: introduce new spinlock to protect
struct link_req"), there is no longer any need to protect struct
link_req or or any link list by use of bearer_lock. Furthermore,
we have eliminated the need for using bearer_lock during downcalls
(send) from the link to the bearer, since we have ensured that
bearers always have a longer life cycle that their associated links,
and always contain valid data.

So, the only need now for a lock protecting bearers is for guaranteeing
consistency of the bearer list itself. For this, it is sufficient, at
least for the time being, to continue applying 'net_lock´ in write mode.

By removing bearer_lock we also pre-empt introduction of issue b) descibed
in the previous commit "tipc: remove 'links' list from tipc_bearer struct":

"b) When the outer protection from net_lock is gone, taking
    bearer_lock and node_lock in opposite order of method 1) and 2)
    will become an obvious deadlock hazard".

Therefore, we now eliminate the bearer_lock spinlock.

Signed-off-by: Ying Xue <ying.xue@windriver.com>
Reviewed-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>