NFSv4: pnfs_roc() must use cred_fscmp() to compare creds

When comparing two 'struct cred' for equality w.r.t. behaviour under
filesystem access, we need to use cred_fscmp().

Fixes: a52458b4 ("NFS/NFSD/SUNRPC: replace generic creds with 'struct cred'.")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>