fs: use mapping_set_error instead of opencoded set_bit

The mapping_set_error() helper sets the correct AS_ flag for the mapping
so there is no reason to open code it.  Use the helper directly.

[akpm@linux-foundation.org: be honest about conversion from -ENXIO to -EIO]
Link: http://lkml.kernel.org/r/20160912111608.2588-2-mhocko@kernel.org
Signed-off-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/ext4/page-io.c

@@ -88,7 +88,7 @@ static void ext4_finish_bio(struct bio *bio)
 
 		if (bio->bi_error) {
 			SetPageError(page);
-			set_bit(AS_EIO, &page->mapping->flags);
+			mapping_set_error(page->mapping, -EIO);
 		}
 		bh = head = page_buffers(page);
 		/*