pagevec: Initialise folio_batch->percpu_pvec_drained

When UBSAN is enabled, it reports an invalid value in __pagevec_release()
when accessing pvec->percpu_pvec_drained, which is simply whatever
garbage was on the stack.  Initialise it when initialising the rest of
the folio_batch.

Fixes: 1033179 ("pagevec: Add folio_batch")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

include/linux/pagevec.h

@@ -111,6 +111,7 @@ static_assert(offsetof(struct pagevec, pages) ==
 static inline void folio_batch_init(struct folio_batch *fbatch)
 {
 	fbatch->nr = 0;
+	fbatch->percpu_pvec_drained = false;
 }
 
 static inline unsigned int folio_batch_count(struct folio_batch *fbatch)