Revert "SUNRPC: Remove unreachable error condition"

Revert "SUNRPC: Remove unreachable error condition"

This reverts commit 079b1b1bf67d57d3af767492ba7684e4bd2eb5c2.

The assumption that it is impossible to return an ERR pointer from
rpc_run_task() no longer holds due to commit 4c22bccac1fd ("SUNRPC:
Handle allocation failure in rpc_new_task()").

Fixes: 4c22bccac1fd ('SUNRPC: Handle allocation failure in rpc_new_task()')
Fixes: 079b1b1bf67d ('SUNRPC: Remove unreachable error condition')
Signed-off-by: Dan Aloni <dan.aloni@vastdata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>