nfsd: quiet compile warning

Stephen Rothwell said:
"Today's linux-next build (powerpc ppc64_defconfig) produced this new
warning:

fs/nfsd/nfs4state.c: In function 'EXPIRED_STATEID':
fs/nfsd/nfs4state.c:2757: warning: comparison of distinct pointer types lacks a cast

Caused by commit 78155ed7 ("nfsd4:
distinguish expired from stale stateids")."
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>
Reported-by: NStephen Rothwell <sfr@canb.auug.org.au>

fs/nfsd/nfs4state.c

@@ -2754,7 +2754,7 @@ EXPIRED_STATEID(stateid_t *stateid)
 {
 	if (time_before((unsigned long)boot_time,
 			((unsigned long)stateid->si_boot)) &&
-	    time_before((stateid->si_boot + lease_time), get_seconds())) {
+	    time_before((unsigned long)(stateid->si_boot + lease_time), get_seconds())) {
 		dprintk("NFSD: expired stateid (%08x/%08x/%08x/%08x)!\n",
 			stateid->si_boot, stateid->si_stateownerid,
 			stateid->si_fileid, stateid->si_generation);