give readdir(2)/getdents(2)/etc. uniform exclusion with lseek()

same as read() on regular files has, and for the same reason.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/open.c b/fs/open.c
index 17cb6b1..938a658 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -713,7 +713,7 @@
 	}
 
 	/* POSIX.1-2008/SUSv4 Section XSI 2.9.7 */
-	if (S_ISREG(inode->i_mode))
+	if (S_ISREG(inode->i_mode) || S_ISDIR(inode->i_mode))
 		f->f_mode |= FMODE_ATOMIC_POS;
 
 	f->f_op = fops_get(inode->i_fop);