nfsd: rpc_peeraddr2str needs rcu lock

[ Upstream commit 05570a2b ]

I'm not even sure cl_xprt can change here, but we're getting "suspicious
RCU usage" warnings, and other rpc_peeraddr2str callers are taking the
rcu lock.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>