NFS: fix unsigned comparison in nfs4_create_sec_client

fs/nfs/nfs4namespace.c: In function ‘nfs4_create_sec_client’:
fs/nfs/nfs4namespace.c:171:2: error: comparison of unsigned expression < 0 is always false [-Werror=type-limits]

Introduced by commit 72de53ec
"NFS: Do secinfo as part of lookup"

Signed-off-by: Benny Halevy <bhalevy@tonian.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>