drbd: Fixed a race condition between detach and start of resync

drbd_state_lock() is only there to serialize cluster wide state
changes. Testing the local disk state needs to happen while
holding the global_state_lock.

Otherwise you might see something like this (Oct 6 on kugel)
14:20:24 drbd0: conn( WFSyncUUID -> Connected ) disk( Inconsistent -> Failed )
14:20:24 drbd0: helper command: /sbin/drbdadm before-resync-target minor-0 exit code 0 (0x0)
14:20:24 drbd0: conn( Connected -> SyncTarget ) disk( Failed -> Inconsistent )

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>