Custom BGM is fixed

Based on this commit hrydgard#5828 on improvements to sceMp3
Custom BGM can be update now. Test on "Miku" is just all right :)

Core/HLE/sceIo.cpp

@@ -1937,7 +1937,26 @@ u32 sceIoDread(int id, u32 dirent_addr) {
 
 		strncpy(entry->d_name, info.name.c_str(), 256);
 		entry->d_name[255] = '\0';
-		entry->d_private = 0xC0DEBABE;
+		// write d_private for supporting Custom BGM
+		// ref JPCSP https://code.google.com/p/jpcsp/source/detail?r=3468
+		if (Memory::IsValidAddress(entry->d_private)){
+			if (sceKernelGetCompiledSdkVersion() <= 0x0307FFFF){
+				// d_private is pointing to an area of unknown size
+				// - [0..12] "8.3" file name (null-terminated), could be empty.
+				// - [13..???] long file name (null-terminated)
+				strncpy((char*)Memory::GetPointer(entry->d_private + 13), (const char*)entry->d_name, ARRAY_SIZE(entry->d_name));
+			}
+			else {
+				// d_private is pointing to an area of total size 1044
+				// - [0..3] size of area
+				// - [4..19] "8.3" file name (null-terminated), could be empty.
+				// - [20..???] long file name (null-terminated)
+				auto size = Memory::Read_U32(entry->d_private);
+				if (size >= 1044) {
+					strncpy((char*)Memory::GetPointer(entry->d_private + 20), (const char*)entry->d_name, ARRAY_SIZE(entry->d_name));
+				}
+			}
+		}
 		DEBUG_LOG(SCEIO, "sceIoDread( %d %08x ) = %s", id, dirent_addr, entry->d_name);
 
 		// TODO: Improve timing.  Only happens on the *first* entry read, ms and umd.