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>