selftests/powerpc: Fix strncpy usage

There is a buffer overflow in dscr_inherit_test.c test. In main(), strncpy()'s
third argument is the length of the source, not the size of the destination
buffer, which makes strncpy() behaves like strcpy(), causing a buffer overflow
if argv[0] is bigger than LEN_MAX (100).

This patch maps 'prog' to the argv[0] memory region, removing the static
allocation and the LEN_MAX size restriction.

Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

tools/testing/selftests/powerpc/dscr/dscr_inherit_exec_test.c

@@ -19,7 +19,7 @@
  */
 #include "dscr.h"
 
-static char prog[LEN_MAX];
+static char *prog;
 
 static void do_exec(unsigned long parent_dscr)
 {
@@ -104,6 +104,6 @@ int main(int argc, char *argv[])
 		exit(1);
 	}
 
-	strncpy(prog, argv[0], strlen(argv[0]));
+	prog = argv[0];
 	return test_harness(dscr_inherit_exec, "dscr_inherit_exec_test");
 }