SUNRPC: Add an IS_ERR() check back to where it was

This IS_ERR() check was deleted during in a cleanup because, at the time,
the rpcb_call_async() function could not return an error pointer.  That
changed in commit 25cf32ad ("SUNRPC: Handle allocation failure in
rpc_new_task()") and now it can return an error pointer.  Put the check
back.

A related revert was done in commit 13bd9014 ("Revert "SUNRPC:
Remove unreachable error condition"").

Fixes: 037e910b ("SUNRPC: Remove unreachable error condition in rpcb_getport_async()")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>