Convert enable_bearer() to RCU locking with dev_get_by_name().

Based on a similar changeset in commit 840a185d ["aoe: remove
dev_base_lock use from aoecmd_cfg_pkts()"] -- quoting that:

  "dev_base_lock is the legacy way to lock the device list,
   and is planned to disappear. (writers hold RTNL, readers
   hold RCU lock)"
Signed-off-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>