fsdax: dedupe should compare the min of two iters' length

In an dedupe comparison iter loop, the length of iomap_iter decreases
because it implies the remaining length after each iteration.

The dedupe command will fail with -EIO if the range is larger than one 
page size and not aligned to the page size.  Also report warning in dmesg:

[ 4338.498374] ------------[ cut here ]------------
[ 4338.498689] WARNING: CPU: 3 PID: 1415645 at fs/iomap/iter.c:16 
...

The compare function should use the min length of the current iters,
not the total length.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 0e79e37 ("fsdax: dedupe: iter two files at the same time")
Signed-off-by: Shiyang Ruan <[email protected]>
Reviewed-by: Darrick J. Wong <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

fs/dax.c

@@ -2027,8 +2027,8 @@ int dax_dedupe_file_range_compare(struct inode *src, loff_t srcoff,
 
 	while ((ret = iomap_iter(&src_iter, ops)) > 0 &&
 	       (ret = iomap_iter(&dst_iter, ops)) > 0) {
-		compared = dax_range_compare_iter(&src_iter, &dst_iter, len,
-						  same);
+		compared = dax_range_compare_iter(&src_iter, &dst_iter,
+				min(src_iter.len, dst_iter.len), same);
 		if (compared < 0)
 			return ret;
 		src_iter.processed = dst_iter.processed = compared;