sys_ipc_try_send: set return value for sys_ipc_recv

[HeyiSun]

The function is here: https://github.com/SimpCosm/6.828/blob/6a0a24dce33484f545f7a883b9b99fc190f25c3f/lab4/code/kern/syscall.c#L314-L357

I think you need e->env_tf.tf_regs.reg_eax = 0;. This makes sure that the sys_ipc_recv called by destination Env returns 0 on success IPC.