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>
diff --git a/fs/buffer.c b/fs/buffer.c
index 7dad871..b205a62 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -351,7 +351,7 @@
 		set_buffer_uptodate(bh);
 	} else {
 		buffer_io_error(bh, ", lost async page write");
-		set_bit(AS_EIO, &page->mapping->flags);
+		mapping_set_error(page->mapping, -EIO);
 		set_buffer_write_io_error(bh);
 		clear_buffer_uptodate(bh);
 		SetPageError(page);
@@ -3249,7 +3249,7 @@
 	bh = head;
 	do {
 		if (buffer_write_io_error(bh) && page->mapping)
-			set_bit(AS_EIO, &page->mapping->flags);
+			mapping_set_error(page->mapping, -EIO);
 		if (buffer_busy(bh))
 			goto failed;
 		bh = bh->b_this_page;