iomap: Inline __iomap_zero_iter into its caller

To make the merge easier, replicate the inlining of __iomap_zero_iter()
into iomap_zero_iter() that is currently in the nvdimm tree.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

fs/iomap/buffered-io.c

@@ -895,13 +895,32 @@ iomap_file_unshare(struct inode *inode, loff_t pos, loff_t len,
 }
 EXPORT_SYMBOL_GPL(iomap_file_unshare);
 
-static s64 __iomap_zero_iter(struct iomap_iter *iter, loff_t pos, u64 length)
+static loff_t iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
 {
+	struct iomap *iomap = &iter->iomap;
+	const struct iomap *srcmap = iomap_iter_srcmap(iter);
+	loff_t pos = iter->pos;
+	loff_t length = iomap_length(iter);
+	loff_t written = 0;
+
+	/* already zeroed?  we're done. */
+	if (srcmap->type == IOMAP_HOLE || srcmap->type == IOMAP_UNWRITTEN)
+		return length;
+
+	do {
 		struct folio *folio;
 		int status;
 		size_t offset;
 		size_t bytes = min_t(u64, SIZE_MAX, length);
 
+		if (IS_DAX(iter->inode)) {
+			s64 tmp = dax_iomap_zero(pos, bytes, iomap);
+			if (tmp < 0)
+				return tmp;
+			bytes = tmp;
+			goto good;
+		}
+
 		status = iomap_write_begin(iter, pos, bytes, &folio);
 		if (status)
 			return status;
@@ -913,30 +932,10 @@ static s64 __iomap_zero_iter(struct iomap_iter *iter, loff_t pos, u64 length)
 		folio_zero_range(folio, offset, bytes);
 		folio_mark_accessed(folio);
 
-	return iomap_write_end(iter, pos, bytes, bytes, folio);
-}
-
-static loff_t iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
-{
-	struct iomap *iomap = &iter->iomap;
-	const struct iomap *srcmap = iomap_iter_srcmap(iter);
-	loff_t pos = iter->pos;
-	loff_t length = iomap_length(iter);
-	loff_t written = 0;
-
-	/* already zeroed?  we're done. */
-	if (srcmap->type == IOMAP_HOLE || srcmap->type == IOMAP_UNWRITTEN)
-		return length;
-
-	do {
-		s64 bytes;
-
-		if (IS_DAX(iter->inode))
-			bytes = dax_iomap_zero(pos, length, iomap);
-		else
-			bytes = __iomap_zero_iter(iter, pos, length);
-		if (bytes < 0)
-			return bytes;
+		bytes = iomap_write_end(iter, pos, bytes, bytes, folio);
+good:
+		if (WARN_ON_ONCE(bytes == 0))
+			return -EIO;
 
 		pos += bytes;
 		length -= bytes;