BACKPORT: loop: Factor out setting loop device size

This code is used repeatedly.

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 5795b6f5607f7e4db62ddea144727780cb351a9b)
Bug: 148607611
Change-Id: I332d05033e13b6c9f75f9d959a4682135bd6c53c