NFS: Fix up a sparse warning

sparse is warning about an incorrect RCU dereference.
fs/nfs/dir.c:2965:56: warning: incorrect type in argument 1 (different address spaces)
fs/nfs/dir.c:2965:56:    expected struct cred const *
fs/nfs/dir.c:2965:56:    got struct cred const [noderef] __rcu *const cred

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>