nfsd: close potential race between delegation break and laundromat

Bruce says:

    There's also a preexisting expire_client/laundromat vs break race:

    - expire_client/laundromat adds a delegation to its local
      reaplist using the same dl_recall_lru field that a delegation
      uses to track its position on the recall lru and drops the
      state lock.

    - a concurrent break_lease adds the delegation to the lru.

    - expire/client/laundromat then walks it reaplist and sees the
      lru head as just another delegation on the list....

Fix this race by checking the dl_time under the state_lock. If we find
that it's not 0, then we know that it has already been queued to the LRU
list and that we shouldn't queue it again.

In the case of destroy_client, we must also ensure that we don't hit
similar races by ensuring that we don't move any delegations to the
reaplist with a dl_time of 0. Just bump the dl_time by one before we
drop the state_lock. We're destroying the delegations anyway, so a 1s
difference there won't matter.

The fault injection code also requires a bit of surgery here:

First, in the case of nfsd_forget_client_delegations, we must prevent
the same sort of race vs. the delegation break callback. For that, we
just increment the dl_time to ensure that a delegation callback can't
race in while we're working on it.

We can't do that for nfsd_recall_client_delegations, as we need to have
it actually queue the delegation, and that won't happen if we increment
the dl_time. The state lock is held over that function, so we don't need
to worry about these sorts of races there.

There is one other potential bug nfsd_recall_client_delegations though.
Entries on the victims list are not dequeued before calling
nfsd_break_one_deleg. That's a potential list corruptor, so ensure that
we do that there.

Reported-by: "J. Bruce Fields" <bfields@fieldses.org>
Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

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