bnxt: add a missing rcu synchronization

BugLink: https://bugs.launchpad.net/bugs/1878232

commit e5f6f564 upstream.

Add a missing synchronize_net() call to avoid potential use after free,
since we explicitly call napi_hash_del() to factorize the RCU grace
period.

Fixes: c0c050c5 ("bnxt_en: New Broadcom ethernet driver.")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Michael Chan <michael.chan@broadcom.com>
Acked-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ian May <ian.may@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/net/ethernet/broadcom/bnxt/bnxt.c

@@ -4250,6 +4250,10 @@ static void bnxt_del_napi(struct bnxt *bp)
 		napi_hash_del(&bnapi->napi);
 		netif_napi_del(&bnapi->napi);
 	}
+	/* We called napi_hash_del() before netif_napi_del(), we need
+	 * to respect an RCU grace period before freeing napi structures.
+	 */
+	synchronize_net();
 }
 
 static void bnxt_init_napi(struct bnxt *bp)