fix d_walk()/non-delayed __d_free() race

fix d_walk()/non-delayed __d_free() race

Orabug: 25256943

[ Upstream commit 3d56c25e3bb0726a5c5e16fc2d9e38f8ed763085 ]

Ascend-to-parent logics in d_walk() depends on all encountered child
dentries not getting freed without an RCU delay.  Unfortunately, in
quite a few cases it is not true, with hard-to-hit oopsable race as
the result.

Fortunately, the fix is simiple; right now the rule is "if it ever
been hashed, freeing must be delayed" and changing it to "if it
ever had a parent, freeing must be delayed" closes that hole and
covers all cases the old rule used to cover.  Moreover, pipes and
sockets remain _not_ covered, so we do not introduce RCU delay in
the cases which are the reason for having that delay conditional
in the first place.

Cc: stable@vger.kernel.org # v3.2+ (and watch out for __d_materialise_dentry())
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
(cherry picked from commit 7296467cfbe04536b8ccec5345a1fd0a2cab1cdc)
Signed-off-by: Dhaval Giani <dhaval.giani@oracle.com>