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: 10331795 ("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>