iomap: Remove unnecessary test from iomap_release_folio()

The check for the folio being under writeback is unnecessary; the caller
has checked this and the folio is locked, so the folio cannot be under
writeback at this point.

The comment is somewhat misleading in that it talks about one specific
situation in which we can see a dirty folio.  There are others, so change
the comment to explain why we can't release the iomap_page.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

fs/iomap/buffered-io.c

@@ -483,12 +483,11 @@ bool iomap_release_folio(struct folio *folio, gfp_t gfp_flags)
 			folio_size(folio));
 
 	/*
-	 * mm accommodates an old ext3 case where clean folios might
-	 * not have had the dirty bit cleared.  Thus, it can send actual
-	 * dirty folios to ->release_folio() via shrink_active_list();
-	 * skip those here.
+	 * If the folio is dirty, we refuse to release our metadata because
+	 * it may be partially dirty.  Once we track per-block dirty state,
+	 * we can release the metadata if every block is dirty.
 	 */
-	if (folio_test_dirty(folio) || folio_test_writeback(folio))
+	if (folio_test_dirty(folio))
 		return false;
 	iomap_page_release(folio);
 	return true;