loop: use vfs_getattr_nosec for accurate file size

Use vfs_getattr_nosec() in lo_calculate_size() for getting the file
size, rather than just read the cached inode size via i_size_read().
This provides better results than cached inode data, particularly for
network filesystems where metadata may be stale.

Signed-off-by: Rajeev Mishra <rajeevm@hpe.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20250818184821.115033-3-rajeevm@hpe.com


[axboe: massage commit message]
Signed-off-by: Jens Axboe <axboe@kernel.dk>