make cursors NORCU

All it requires is making sure that d_walk() will skip *all*
CURSOR dentries, even if somebody passes it one as an argument.

Cursors are negative and unhashed all along, never get added to
LRU or to shrink lists and no RCU references via ->d_sib are
possible for those - dentry_unlist() makes sure that no killed
dentry has ->d_sib.next left pointing to a cursor.

Seeing that a cursor is allocated every time we open a directory
on autofs, debugfs, devpts, etc., avoiding an RCU delay when such
opened files get closed is attractive...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>