nfsd4: fix nfs4 stateid leak

commit cf9182e90b2af04245ac4fae497fe73fc71285b4 upstream.

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>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>