floppy_open() tries to make blocking read/write opens perform a fresh
media-change check by clearing drive_state[drive].last_checked before
calling disk_check_media_change().  That timestamp is also updated by
disk_change() when another FDC operation observes that the disk-change
line is clear.

The worker path that calls disk_change() is not serialized by the
floppy_mutex/open_lock pair held by floppy_open().  A worker can therefore
store a recent jiffies value after floppy_open() stores zero and before the
synchronous disk_check_media_change() call reaches floppy_check_events().
The checkfreq throttle can then treat the media-change state as fresh and
skip the hardware poll that the open path explicitly tried to force.

Use the block layer's clearing mask instead of a racy timestamp sentinel.
When DISK_EVENT_MEDIA_CHANGE is being synchronously cleared, bypass the
checkfreq throttle and poll the drive.  Periodic event checks still use
last_checked to avoid unnecessary polling, and floppy_open() no longer
needs to write last_checked itself.

Signed-off-by: Cen Zhang <zzzccc427@gmail.com>