iomap: wait for batched folios to be stable in __iomap_get_folio

__iomap_get_folio needs to wait for writeback to finish if the file
requires folios to be stable for writes.  For the regular path this is
taken care of by __filemap_get_folio, but for the newly added batch
lookup it has to be done manually.

This fixes xfs/131 failures when running on PI-capable hardware.

Fixes: 395ed1ef00 ("iomap: optional zero range dirty folio processing")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20260113153943.3323869-1-hch@lst.de
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/iomap/buffered-io.c

@@ -851,6 +851,7 @@ static struct folio *__iomap_get_folio(struct iomap_iter *iter,
 		}
 
 		folio_get(folio);
+		folio_wait_stable(folio);
 		return folio;
 	}