fio: Add advise THP option to mmap engine

fio: Add advise THP option to mmap engine

The Linux specific transparent hugepage memory advisory has potentially
significant implications for how the memory management behaves. If the
platform supports it, add a new mmap ioengine specific option that advises
HUGEPAGE on an mmap'ed range. The option availability is detected during
configure. If the option is set, fio can test THP when used with private
anonymous memory (i.e. mmap /dev/zero).

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>