mm: fix assertion in folio_end_read()

We only need to assert that the uptodate flag is clear if we're going to
set it.  This hasn't been a problem before now because we have only used
folio_end_read() when completing with an error, but it's convenient to use
it in squashfs if we discover the folio is already uptodate.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Cc: Phillip Lougher <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Matthew Wilcox (Oracle)

mm/filemap.c

@@ -1523,7 +1523,7 @@ void folio_end_read(struct folio *folio, bool success)
 	/* Must be in bottom byte for x86 to work */
 	BUILD_BUG_ON(PG_uptodate > 7);
 	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
-	VM_BUG_ON_FOLIO(folio_test_uptodate(folio), folio);
+	VM_BUG_ON_FOLIO(success && folio_test_uptodate(folio), folio);
 
 	if (likely(success))
 		mask |= 1 << PG_uptodate;