LU-641 obdclass: use rwlock for obd_dev_lock

LU-641 obdclass: use rwlock for obd_dev_lock

In target_handle_connect() it grabs obd_dev_lock to find obd_device
by target uuid. This causes lock contention if there are two many connection
requests on the same time. Based on obd_dev_lock is a read-mostly lock, it
is much reasonable to revise it as a rwlock.

Change-Id: I7f936d6182df9e7ed46b8a1b6fb259226188acbc
Signed-off-by: Jinshan Xiong <jay@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/1294
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>