get rid of assorted nameidata-related debris

pointless forward declarations, stale comments

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/include/linux/sched.h b/include/linux/sched.h
index f6c9b69..a1158c9 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -132,6 +132,7 @@
 struct perf_event_context;
 struct blk_plug;
 struct filename;
+struct nameidata;
 
 #define VMACACHE_BITS 2
 #define VMACACHE_SIZE (1U << VMACACHE_BITS)