AK+Kernel: Return KString from MACAddress::to_string() in the Kernel

This lets us safely handle allocation failure.

AK/MACAddress.h

@@ -9,10 +9,15 @@
 #include <AK/AllOf.h>
 #include <AK/Array.h>
 #include <AK/Assertions.h>
-#include <AK/String.h>
 #include <AK/Types.h>
 #include <AK/Vector.h>
 
+#ifdef KERNEL
+#    include <Kernel/KString.h>
+#else
+#    include <AK/String.h>
+#endif
+
 class [[gnu::packed]] MACAddress {
     static constexpr size_t s_mac_address_length = 6u;
 
@@ -53,10 +58,17 @@ public:
         return true;
     }
 
+#ifdef KERNEL
+    ErrorOr<NonnullOwnPtr<Kernel::KString>> to_string() const
+    {
+        return Kernel::KString::formatted("{:02x}:{:02x}:{:02x}:{:02x}:{:02x}:{:02x}", m_data[0], m_data[1], m_data[2], m_data[3], m_data[4], m_data[5]);
+    }
+#else
     String to_string() const
     {
         return String::formatted("{:02x}:{:02x}:{:02x}:{:02x}:{:02x}:{:02x}", m_data[0], m_data[1], m_data[2], m_data[3], m_data[4], m_data[5]);
     }
+#endif
 
     static Optional<MACAddress> from_string(StringView string)
     {

Kernel/GlobalProcessExposed.cpp

@@ -45,7 +45,8 @@ private:
             auto obj = array.add_object();
             obj.add("name", adapter.name());
             obj.add("class_name", adapter.class_name());
-            obj.add("mac_address", adapter.mac_address().to_string());
+            auto mac_address = adapter.mac_address().to_string().release_value_but_fixme_should_propagate_errors();
+            obj.add("mac_address", mac_address->view());
             if (!adapter.ipv4_address().is_zero()) {
                 auto ipv4_address = adapter.ipv4_address().to_string().release_value_but_fixme_should_propagate_errors();
                 obj.add("ipv4_address", ipv4_address->view());
@@ -81,7 +82,8 @@ private:
         JsonArraySerializer array { builder };
         arp_table().for_each([&](const auto& it) {
             auto obj = array.add_object();
-            obj.add("mac_address", it.value.to_string());
+            auto mac_address = it.value.to_string().release_value_but_fixme_should_propagate_errors();
+            obj.add("mac_address", mac_address->view());
             auto ip_address = it.key.to_string().release_value_but_fixme_should_propagate_errors();
             obj.add("ip_address", ip_address->view());
         });