afs: convert pagevec to folio_batch in afs_extend_writeback()

Patch series "Remove pagevecs".

Removes a folio->page->folio conversion for each folio that's involved. 
More importantly, removes one of the last few uses of a pagevec.

Link: https://lkml.kernel.org/r/20230621164557.3510324-1-willy@infradead.org
Link: https://lkml.kernel.org/r/20230621164557.3510324-2-willy@infradead.org


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