tools/selftest/vm: allow choosing mem size and page size in map_hugetlb

map_hugetlb maps 256Mbytes of memory with default hugepage size.

This patch allows the user to pass the size and page shift as an
argument in order to use different size and page size.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>