SUNRPC: RPC level errors should set task->tk_rpc_status

Fix up a case in call_encode() where we're failing to set
task->tk_rpc_status when an RPC level error occurred.

Fixes: 9c5948c2

 ("SUNRPC: task should be exit if encode return EKEYEXPIRED more times")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>