NFS: Fix a double page unlock

Since commit 0bcbf039, nfs_readpage_release() has been used to
unlock the page in the read code.

Fixes: 0bcbf039 ("nfs: handle request add failure properly")
Cc: stable@vger.kernel.org # v4.5+
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>