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