NFS: fix mount/umount race in nlmclnt.

NFS: fix mount/umount race in nlmclnt.

commit 4a9be28c45bf02fa0436808bb6c0baeba30e120e upstream.

If the last NFSv3 unmount from a given host races with a mount from the
same host, we can destroy an nlm_host that is still in use.

Specifically nlmclnt_lookup_host() can increment h_count on
an nlm_host that nlmclnt_release_host() has just successfully called
refcount_dec_and_test() on.
Once nlmclnt_lookup_host() drops the mutex, nlm_destroy_host_lock()
will be called to destroy the nlmclnt which is now in use again.

The cause of the problem is that the dec_and_test happens outside the
locked region.  This is easily fixed by using
refcount_dec_and_mutex_lock().

Fixes: 8ea6ecc8b075 ("lockd: Create client-side nlm_host cache")
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
[bwh: Backported to 3.16: use atomic instead of refcount API]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>