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.