fuse: don't invalidate attrs when not using atime

Various read operations (e.g. readlink, readdir) invalidate the cached
attrs for atime changes.  This patch adds a new function
'fuse_invalidate_atime', which checks for a read-only super block and
avoids the attr invalidation in that case.

Signed-off-by: Andrew Gallagher <andrewjcg@fb.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 7e70506..d53af8f 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -687,7 +687,7 @@
 		SetPageUptodate(page);
 	}
 
-	fuse_invalidate_attr(inode); /* atime changed */
+	fuse_invalidate_atime(inode);
  out:
 	unlock_page(page);
 	return err;
@@ -716,7 +716,7 @@
 			fuse_read_update_size(inode, pos,
 					      req->misc.read.attr_ver);
 		}
-		fuse_invalidate_attr(inode); /* atime changed */
+		fuse_invalidate_atime(inode);
 	}
 
 	for (i = 0; i < req->num_pages; i++) {