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/fuse_i.h b/fs/fuse/fuse_i.h
index 7d27309..dc44b9e 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -788,6 +788,8 @@
 
 void fuse_invalidate_entry_cache(struct dentry *entry);
 
+void fuse_invalidate_atime(struct inode *inode);
+
 /**
  * Acquire reference to fuse_conn
  */