nfs/localio: restore creds before releasing pageio data

In the Linux kernel, the following vulnerability has been resolved:

nfs/localio: restore creds before releasing pageio data

Otherwise if the nfsd filecache code releases the nfsd_file immediately, it can trigger the BUG_ON(cred == current->cred) in __put_cred() when it puts the nfsd_file->nf_file->f-cred.