autofs races

autofs races

* make autofs4_expire_indirect() skip the dentries being in process of
expiry
* do *not* mess with list_move(); making sure that dentry with
AUTOFS_INF_EXPIRING are not picked for expiry is enough.
* do not remove NO_RCU when we set EXPIRING, don't bother with smp_mb()
there.  Clear it at the same time we clear EXPIRING.  Makes a bunch of
tests simpler.
* rename NO_RCU to WANT_EXPIRE, which is what it really is.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Orabug: 26032471
(cherry picked from commit ea01a18494b3d7a91b2f1f2a6a5aaef4741bc294)
Signed-off-by: Mingming Cao <mingming.cao@oracle.com>
Reviewed-by: Shirley Ma  <shirley.ma@oracle.com>