net: use strnstr() for lwip_strnstr()

Using strstr() instead of strnstr() creates a security concern.

Fixes: 1c41a7afaa15 ("net: lwip: build lwIP")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
diff --git a/lib/lwip/u-boot/arch/cc.h b/lib/lwip/u-boot/arch/cc.h
index de13884..6104c29 100644
--- a/lib/lwip/u-boot/arch/cc.h
+++ b/lib/lwip/u-boot/arch/cc.h
@@ -34,7 +34,7 @@
 	       x, __LINE__, __FILE__); } while (0)
 
 #define atoi(str) (int)dectoul(str, NULL)
-#define lwip_strnstr(a, b, c)  strstr(a, b)
+#define lwip_strnstr(a, b, c)  strnstr(a, b, c)
 
 #define LWIP_ERR_T int
 #define LWIP_CONST_CAST(target_type, val) ((target_type)((uintptr_t)val))