folio_batch: rename PAGEVEC_SIZE to FOLIO_BATCH_SIZE

folio_batch: rename PAGEVEC_SIZE to FOLIO_BATCH_SIZE

struct pagevec no longer exists.  Rename the macro appropriately.

Link: https://lkml.kernel.org/r/20260225-pagevec_cleanup-v2-4-716868cc2d11@columbia.edu
Signed-off-by: Tal Zussman <tz2294@columbia.edu>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Acked-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>