erofs-utils: lib: fix erofs_io_sendfile() again

The number of bytes written should be returned.

Fixes: 29466e7f1cbf ("erofs-utils: lib: fix erofs_io_sendfile()")
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20251017112002.1254940-1-hsiangkao@linux.alibaba.com