nfsd: ensure that delegation stateid hash references are only put once

It's possible that a DELEGRETURN could race with (e.g.) client expiry,
in which case we could end up putting the delegation hash reference more
than once.

Have unhash_delegation_locked return a bool that indicates whether it
was already unhashed. In the case of destroy_delegation we only
conditionally put the hash reference if that returns true.

The other callers of unhash_delegation_locked call it while walking
list_heads that shouldn't yet be detached. If we find that it doesn't
return true in those cases, then throw a WARN_ON as that indicates that
we have a partially hashed delegation, and that something is likely very
wrong.

Tested-by: Andrew W Elble <[email protected]>
Tested-by: Anna Schumaker <[email protected]>
Signed-off-by: Jeff Layton <[email protected]>
Cc: [email protected]
Signed-off-by: J. Bruce Fields <[email protected]>

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