loop: remove extra variable in lo_req_flush

loop: remove extra variable in lo_req_flush

The local variable file is used to pass it to the vfs_fsync(). We can
get away with using lo->lo_backing_file instead of storing in a local
variable which is not used anywhere else.

No functional change in this patch.

Signed-off-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Link: https://lore.kernel.org/r/20220215213310.7264-4-kch@nvidia.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>