selftests/powerpc: Skip the subpage_prot tests if the syscall is unavailable

The subpage_prot syscall is only functional when the system is using
the Hash MMU. Since commit 5b2b8071 ("powerpc/mm: Invalidate
subpage_prot() system call on radix platforms") it returns ENOENT when
the Radix MMU is active. Currently this just makes the test fail.

Additionally the syscall is not available if the kernel is built with
4K pages, or if CONFIG_PPC_SUBPAGE_PROT=n, in which case it returns
ENOSYS because the syscall is missing entirely.

So check explicitly for ENOENT and ENOSYS and skip if we see either of
those.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>