s390/dasd: protect device queue against concurrent access

In the Linux kernel, the following vulnerability has been resolved: s390/dasd: protect device queue against concurrent access In dasd_profile_start() the amount of requests on the device queue are counted. The access to the device queue is unprotected against concurrent access. With a lot of parallel I/O, especially with alias devices enabled, the device queue can change while dasd_profile_start() is accessing the queue. In the worst case this leads to a kernel panic due to incorrect pointer accesses. Fix this by taking the device lock before accessing the queue and counting the requests. Additionally the check for a valid profile data pointer can be done earlier to avoid unnecessary locking in a hot path.