docs: document iomap writeback's iomap_finish_folio_write() requirement

Document that iomap_finish_folio_write() must be called after writeback
on the range completes.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Link: https://patch.msgid.link/20251111193658.3495942-4-joannelkoong@gmail.com
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Documentation/filesystems/iomap/operations.rst

@@ -361,6 +361,9 @@ The fields are as follows:
     delalloc reservations to avoid having delalloc reservations for
     clean pagecache.
     This function must be supplied by the filesystem.
+    If this succeeds, iomap_finish_folio_write() must be called once writeback
+    completes for the range, regardless of whether the writeback succeeded or
+    failed.
 
   - ``writeback_submit``: Submit the previous built writeback context.
     Block based file systems should use the iomap_ioend_writeback_submit

include/linux/iomap.h

@@ -435,6 +435,10 @@ struct iomap_writeback_ops {
 	 * An existing mapping from a previous call to this method can be reused
 	 * by the file system if it is still valid.
 	 *
+	 * If this succeeds, iomap_finish_folio_write() must be called once
+	 * writeback completes for the range, regardless of whether the
+	 * writeback succeeded or failed.
+	 *
 	 * Returns the number of bytes processed or a negative errno.
 	 */
 	ssize_t (*writeback_range)(struct iomap_writepage_ctx *wpc,