mkfs: fix file size setting when interpreting a protofile

mkfs: fix file size setting when interpreting a protofile

When we're copying a regular file into the filesystem, we should set the
size of the new file to the size indicated by the stat data, not the
highest offset written, because we now use SEEK_DATA/HOLE to ignore
sparse regions.

Fixes: 73fb78e5ee8940 ("mkfs: support copying in large or sparse files")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>