repart: copy --copy-from= partitions at their offset

With --copy-from=, we seek the source image to each partition before
copying its blocks. All partitions that are copied from the same image
share one file offset. Let's pass the partition offset to
copy_bytes_full() instead, so we don't have to move the file offset.

src/repart/repart.c

@@ -6642,14 +6642,10 @@ static int context_copy_blocks(Context *context) {
                 if (p->copy_blocks_offset == UINT64_MAX)
                         log_info("Copying in '%s' (%s) on block level into future partition %" PRIu64 ".",
                                  p->copy_blocks_path, FORMAT_BYTES(p->copy_blocks_size), p->partno);
-                else {
+                else
                         log_info("Copying in '%s' @ %" PRIu64 " (%s) on block level into future partition %" PRIu64 ".",
                                  p->copy_blocks_path, p->copy_blocks_offset, FORMAT_BYTES(p->copy_blocks_size), p->partno);
 
-                        if (lseek(p->copy_blocks_fd, p->copy_blocks_offset, SEEK_SET) < 0)
-                                return log_error_errno(errno, "Failed to seek to copy blocks offset in %s: %m", p->copy_blocks_path);
-                }
-
                 /* We call copy_bytes_full() instead of copy_file_range() directly, because copy_file_range()
                  * needs to be called with a size limit to allow for progress updates. But we don't want that
                  * for cloning, we want one big massive reflink if possible, and unfortunately we can't know
@@ -6658,7 +6654,7 @@ static int context_copy_blocks(Context *context) {
                  * clone, and then falls back to copying in chunks. */
                 r = copy_bytes_full(
                                 p->copy_blocks_fd,
-                                /* foffset= */ NULL,
+                                /* foffset= */ p->copy_blocks_offset == UINT64_MAX ? NULL : &(uint64_t) { p->copy_blocks_offset },
                                 partition_target_fd(t),
                                 /* toffset= */ NULL,
                                 p->copy_blocks_size,