net: enetc: fix VSI mailbox timeout handling and DMA lifecycle

net: enetc: fix VSI mailbox timeout handling and DMA lifecycle

In the current VSI mailbox implementation, the VSI allocates a DMA buffer
to store the message sent to the PSI. When the PSI receives the message
request from the VSI, the hardware copies the message data from this DMA
buffer to PSI's DMA buffer for processing.

When enetc_msg_vsi_send() times out, two scenarios can occur:

1) Use-after-free: If the hardware hasn't completed message copying when
   the VSI frees the buffer, the hardware may subsequently copy the data
   from freed memory to PSI's DMA buffer.

2) Message race: If PSI hasn't processed the previous message when the
   next message is sent, the VSI may receive the previous message's
   reply, leading to incorrect handling.

To address these issues, implement the following changes:

- Check the mailbox busy status before sending a new message. If the
  mailbox is in busy state, it indicates the previous message is still
  being processed, so return an error immediately.

- Add the 'msg' field to struct enetc_si to preserve the DMA buffer
  information. The caller of enetc_msg_vsi_send() no longer frees the
  DMA buffer. Instead, defer freeing until it is safe to do so (when
  mailbox is not busy on next send).

- Add cleanup in enetc_vf_remove() to free the last message buffer.

This ensures the DMA buffer remains valid during message copying and
prevents message reply mismatches.

Fixes: beb74ac878c8 ("enetc: Add vf to pf messaging support")
Signed-off-by: Wei Fang <wei.fang@nxp.com>
Link: https://patch.msgid.link/20260429081930.3259824-1-wei.fang@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>