nfsd: fix possible badness in FREE_STATEID

In the Linux kernel, the following vulnerability has been resolved:

nfsd: fix possible badness in FREE_STATEID

When multiple FREE_STATEIDs are sent for the same delegation stateid, it can lead to a possible either use-after-free or counter refcount underflow errors.

In nfsd4_free_stateid() under the client lock we find a delegation stateid, however the code drops the lock before calling nfs4_put_stid(), that allows another FREE_STATE to find the stateid again. The first one will proceed to then free the stateid which leads to either use-after-free or decrementing already zeroed counter.