restore mutex_lock() call to blk_mq_freeze_queue_start()

restore mutex_lock() call to blk_mq_freeze_queue_start()

Orabug: 26266901

Over-eager editing for conflict resolution in merge commit
c311ca8a3d9349dfc606cada0d8ca14e58728c9c ("Merge tag 'v4.1.12-70#22913653a12'
into pmem-4.1-merge") resulted in the accidental deletion of two calls to
mutex_lock().  Since the matching calls to mutex_unlock() were correctly left
in the source, this produced a lock imbalance, detectable via lockdep.

This commit restores the missing mutex_lock() calls.

Signed-off-by: Dan Duval <dan.duval@oracle.com>