SUNRPC: Remove unreachable error condition in rpcb_getport_async()

rpcb_getport_async() invokes rpcb_call_async(), which return the value
of rpc_run_task() to "child". Since rpc_run_task() is impossible to
return an ERR pointer, there is no need to add the IS_ERR() condition on
"child" here. So we need to remove it.

Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>