rpmsg: glink: smem: order FIFO read after availability check

glink_smem_rx_peek() reads the RX FIFO payload after the caller has
determined data is available via glink_smem_rx_avail(), which reads the
remote-updated head index. A control dependency between the head read
and the subsequent payload read does not order the two loads, so the
CPU may speculatively read the FIFO before observing the head update
and consume stale data the remote has not yet published.

Add rmb() in glink_smem_rx_peek() before the memcpy_fromio() so the
availability (head) read is ordered ahead of the FIFO payload read,
matching the consumer pattern in
Documentation/core-api/circular-buffers.rst.

Fixes: caf989c350e8 ("rpmsg: glink: Introduce glink smem based transport")
Cc: stable@vger.kernel.org
Signed-off-by: Chunkai Deng <chunkai.deng@oss.qualcomm.com>
Reviewed-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20260618-rpmsg-glink-smem-mb-v1-1-68a026453a69@oss.qualcomm.com
Signed-off-by: Bjorn Andersson <andersson@kernel.org>