SUNRPC: Fix oops in the rpc_xdr_buf event class

Backchannel rpc tasks don't have task->tk_client set, so it's necessary
to check it for NULL before dereferencing.

Fixes: c509f15a ("SUNRPC: Split the xdr_buf event class")
Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>