loop: drop caches if offset or block_size are changed

If we don't drop caches used in old offset or block_size, we can get old data
from new offset/block_size, which gives unexpected data to user.

For example, Martijn found a loopback bug in the below scenario.
1) LOOP_SET_FD loads first two pages on loop file
2) LOOP_SET_STATUS64 changes the offset on the loop file
3) mount is failed due to the cached pages having wrong superblock

Cc: Jens Axboe <[email protected]>
Cc: [email protected]
Reported-by: Martijn Coenen <[email protected]>
Reviewed-by: Bart Van Assche <[email protected]>
Signed-off-by: Jaegeuk Kim <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

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