NFS: Avoid RCU usage in tracepoints

There isn't an obvious way to acquire and release the RCU lock during a
tracepoint, so we can't use the rpc_peeraddr2str() function here.
Instead, rely on the client's cl_hostname, which should have similar
enough information without needing an rcu_dereference().

Reported-by: Dave Jones <davej@codemonkey.org.uk>
Cc: stable@vger.kernel.org # v3.12
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>