zdtm/thp_disable: increase memory size to 2M

Somehow in travis tests the mmap of 8K gets "rounded" down and the actual
VMA starts 4K below the returned address. This causes /proc/pid/smaps
parsing to fail.

Increasing the allocation size to 2M seems to resolve the problem and until
the root cause is identified it looks like a viable workaround.

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>