usrsock_rpmsg_server: Add net_lock to combine get_tx_payload and recvfrom in recvfrom_handler

A dead lock may happen before this patch:
rptun thread (usrsock_rpmsg_recvfrom_handler):
 Gets all tx payload but waiting net_lock in psock_recvfrom
net driver thread (higher priority):
 Receives a packet, holding net_lock, calling usrsock_rpmsg_send_event
 But no tx buffer left, so rpmsg_send blocks, and won't release net_lock

In short:
Rptun: Hold all `tx_payload_buffer` -> want `net_lock`
Driver: Hold `net_lock` -> want `tx_payload_buffer`

This patch use net_lock to combine get_tx_payload and recvfrom together,
then:
- If it's waiting net_lock, tx payload will not be held, other threads
  may get tx buffer and do their work.
- If net_lock is got, it won't be disturbed before finish receiving,
  then tx payload won't have chance to be blocked before releasing.

Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>

drivers/usrsock/usrsock_rpmsg_server.c

@@ -562,6 +562,13 @@ static int usrsock_rpmsg_recvfrom_handler(FAR struct rpmsg_endpoint *ept,
                      (FAR void *)(ack + 1) + inaddrlen, ret);
             }
 
+          /* Hold net_lock to combine get_tx_payload and recvfrom together.
+           * Otherwise we may keep holding tx buffer when waiting net_lock in
+           * recvfrom, which may block rpmsg and may cause dead lock if
+           * another thread tries to get tx buffer with net_lock held.
+           */
+
+          net_lock();
           while (totlen < buflen &&
                  i < CONFIG_NET_USRSOCK_RPMSG_SERVER_NIOVEC)
             {
@@ -616,6 +623,8 @@ static int usrsock_rpmsg_recvfrom_handler(FAR struct rpmsg_endpoint *ept,
             {
               events |= USRSOCK_EVENT_RECVFROM_AVAIL;
             }
+
+          net_unlock();
         }
     }