autofs4 - fix lockdep splat in autofs

When recursing down the locks when traversing a tree/list in
get_next_positive_dentry() or get_next_positive_subdir() a lock can
change from being nested to being a parent which breaks lockdep. This
patch tells lockdep about what we did.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Ian Kent <raven@themaw.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>