fix all sign-compare warnings

src/modules/dataman/CMakeLists.txt

@@ -35,7 +35,6 @@ px4_add_module(
 	MAIN dataman
 	STACK_MAIN 1200
 	COMPILE_FLAGS
-		-Wno-sign-compare # TODO: fix all sign-compare
 	SRCS
 		dataman.cpp
 	)

src/modules/dataman/dataman.cpp

@@ -808,7 +808,7 @@ static int  _ram_restart(dm_reset_reason reason)
 static int
 _file_restart(dm_reset_reason reason)
 {
-	unsigned offset = 0;
+	int offset = 0;
 	int result = 0;
 	/* We need to scan the entire file and invalidate and data that should not persist after the last reset */