nfsd4: fix nfs4 stateid leak

Processes that open and close multiple files may end up setting this
oo_last_closed_stid without freeing what was previously pointed to.
This can result in a major leak, visible for example by watching the
nfsd4_stateids line of /proc/slabinfo.

Reported-by: Cyril B. <cbay@excellency.fr>
Tested-by: Cyril B. <cbay@excellency.fr>
Cc: stable@vger.kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>