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: J. Bruce Fields <bfields@citi.umich.edu>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>