IOS/FS: Make ConvertResult usable from ES

ES makes extensive use of FS and most of the time returns its error
codes directly.

Source/Core/Core/IOS/FS/FileSystem.cpp

@@ -6,6 +6,7 @@
 
 #include "Common/Assert.h"
 #include "Common/FileUtil.h"
+#include "Core/IOS/Device.h"
 #include "Core/IOS/FS/HostBackend/FS.h"
 
 namespace IOS::HLE::FS
@@ -17,6 +18,15 @@ std::unique_ptr<FileSystem> MakeFileSystem(Location location)
   return std::make_unique<HostFileSystem>(nand_root);
 }
 
+IOS::HLE::ReturnCode ConvertResult(ResultCode code)
+{
+  if (code == ResultCode::Success)
+    return IPC_SUCCESS;
+  // FS error codes start at -100. Since result codes in the enum are listed in the same way
+  // as the IOS codes, we just need to return -100-code.
+  return static_cast<ReturnCode>(-(static_cast<s32>(code) + 100));
+}
+
 FileHandle::FileHandle(FileSystem* fs, Fd fd) : m_fs{fs}, m_fd{fd}
 {
 }

Source/Core/Core/IOS/FS/FileSystem.h

@@ -14,7 +14,11 @@
 
 class PointerWrap;
 
-namespace IOS::HLE::FS
+namespace IOS::HLE
+{
+enum ReturnCode : s32;
+
+namespace FS
 {
 enum class ResultCode
 {
@@ -218,4 +222,8 @@ enum class Location
 
 std::unique_ptr<FileSystem> MakeFileSystem(Location location = Location::Session);
 
-}  // namespace IOS::HLE::FS
+/// Convert a FS result code to an IOS error code.
+IOS::HLE::ReturnCode ConvertResult(ResultCode code);
+
+}  // namespace FS
+}  // namespace IOS::HLE

Source/Core/Core/IOS/FS/FileSystemProxy.cpp

@@ -23,13 +23,6 @@ namespace Device
 {
 using namespace IOS::HLE::FS;
 
-static s32 ConvertResult(ResultCode code)
-{
-  if (code == ResultCode::Success)
-    return IPC_SUCCESS;
-  return -(static_cast<s32>(code) + 100);
-}
-
 static IPCCommandResult GetFSReply(s32 return_value, u64 extra_tb_ticks = 0)
 {
   // According to hardware tests, FS takes at least 2700 TB ticks to reply to commands.