BACKPORT: loop: Remove sector_t truncation checks

sector_t is now always u64, so we don't need to check for truncation.

Signed-off-by: Martijn Coenen <maco@android.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
(cherry picked from commit 083a6a50783ef54256eec3499e6575237e0e3d53)
Bug: 148607611
Change-Id: I1db87e391aca6226d542c5b74e569184ec059073