Revert "SUNRPC: Remove unreachable error condition"

This reverts commit efe57fd5.

The assumption that it is impossible to return an ERR pointer from
rpc_run_task() no longer holds due to commit 25cf32ad ("SUNRPC:
Handle allocation failure in rpc_new_task()").

Fixes: 25cf32ad ('SUNRPC: Handle allocation failure in rpc_new_task()')
Fixes: efe57fd5 ('SUNRPC: Remove unreachable error condition')
Signed-off-by: Dan Aloni <dan.aloni@vastdata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>