Fix percentile_list option

Fix percentile_list option

Commit 802ad4a8 broke the parsing of the percentile list,
by neglecting to change the parser type fo a fio_fp64_t.

Also get rid of the def_percentile_list, just set the
default percentile list as the option default.

Signed-off-by: Jens Axboe <axboe@kernel.dk>