Make {Un}Marshal{Bytes/Unsafe} return remaining buffer.

Change marshal.Marshallable method signatures to return the remaining buffer.
This makes it easier to implement these method manually. Without this, we would
have to manually do buffer shifting which is error prone.

tools/go_marshal/test:benchmark test does not show change in performance.
Additionally fixed some marshalling bugs in fsimpl/fuse.

Updated multiple callpoints to get rid of redundant slice indexing work and
simplified code using this new signature.

Updates #6450

PiperOrigin-RevId: 407857019

pkg/abi/linux/msgqueue.go

@@ -82,15 +82,15 @@ func (b *MsgBuf) SizeBytes() int {
 }
 
 // MarshalBytes implements marshal.Marshallable.MarshalBytes.
-func (b *MsgBuf) MarshalBytes(dst []byte) {
-	b.Type.MarshalUnsafe(dst)
-	b.Text.MarshalBytes(dst[b.Type.SizeBytes():])
+func (b *MsgBuf) MarshalBytes(dst []byte) []byte {
+	dst = b.Type.MarshalUnsafe(dst)
+	return b.Text.MarshalBytes(dst)
 }
 
 // UnmarshalBytes implements marshal.Marshallable.UnmarshalBytes.
-func (b *MsgBuf) UnmarshalBytes(src []byte) {
-	b.Type.UnmarshalUnsafe(src)
-	b.Text.UnmarshalBytes(src[b.Type.SizeBytes():])
+func (b *MsgBuf) UnmarshalBytes(src []byte) []byte {
+	src = b.Type.UnmarshalUnsafe(src)
+	return b.Text.UnmarshalBytes(src)
 }
 
 // MsgInfo is equivelant to struct msginfo. Source: include/uapi/linux/msg.h