NFSv4.x: Fix a refcount leak in nfs_callback_up_net

[ Upstream commit 98b0f80c2396224bbbed81792b526e6c72ba9efa ]

On error, the callers expect us to return without bumping
nn->cb_users[].

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Cc: stable@vger.kernel.org # v3.7+
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>