wireguard: selftests: use normal kernel stack size on ppc64

While at some point it might have made sense to be running these tests
on ppc64 with 4k stacks, the kernel hasn't actually used 4k stacks on
64-bit powerpc in a long time, and more interesting things that we test
don't really work when we deviate from the default (16k). So, we stop
pushing our luck in this commit, and return to the default instead of
the minimum.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: David S. Miller <davem@davemloft.net>