pagevec: rename fbatch_count()

This should always have been called folio_batch_count().

Link: https://lkml.kernel.org/r/20230621164557.3510324-8-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>