iomap: convert iomap_unshare_iter to use large folios

Convert iomap_unshare_iter to create large folios if possible, since the
write and zeroing paths already do that.  I think this got missed in the
conversion of the write paths that landed in 6.6-rc1.

Cc: ritesh.list@gmail.com, willy@infradead.org
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>

fs/iomap/buffered-io.c

@@ -1263,7 +1263,6 @@ static loff_t iomap_unshare_iter(struct iomap_iter *iter)
 	const struct iomap *srcmap = iomap_iter_srcmap(iter);
 	loff_t pos = iter->pos;
 	loff_t length = iomap_length(iter);
-	long status = 0;
 	loff_t written = 0;
 
 	/* don't bother with blocks that are not shared to start with */
@@ -1274,28 +1273,33 @@ static loff_t iomap_unshare_iter(struct iomap_iter *iter)
 		return length;
 
 	do {
-		unsigned long offset = offset_in_page(pos);
-		unsigned long bytes = min_t(loff_t, PAGE_SIZE - offset, length);
 		struct folio *folio;
+		int status;
+		size_t offset;
+		size_t bytes = min_t(u64, SIZE_MAX, length);
 
 		status = iomap_write_begin(iter, pos, bytes, &folio);
 		if (unlikely(status))
 			return status;
-		if (iter->iomap.flags & IOMAP_F_STALE)
+		if (iomap->flags & IOMAP_F_STALE)
 			break;
 
-		status = iomap_write_end(iter, pos, bytes, bytes, folio);
-		if (WARN_ON_ONCE(status == 0))
+		offset = offset_in_folio(folio, pos);
+		if (bytes > folio_size(folio) - offset)
+			bytes = folio_size(folio) - offset;
+
+		bytes = iomap_write_end(iter, pos, bytes, bytes, folio);
+		if (WARN_ON_ONCE(bytes == 0))
 			return -EIO;
 
 		cond_resched();
 
-		pos += status;
-		written += status;
-		length -= status;
+		pos += bytes;
+		written += bytes;
+		length -= bytes;
 
 		balance_dirty_pages_ratelimited(iter->inode->i_mapping);
-	} while (length);
+	} while (length > 0);
 
 	return written;
 }