Once sbull_open has its device structure pointer, it calls del_timer_sync to remove the
“media removal” timer, if any is active. Note that we do not lock the device spinlock
until after the timer has been deleted; doing otherwise invites deadlock if the timer
function runs before we can delete it. With the device locked, we call a kernel func-
tion called check_disk_change to check whether a media change has happened.
