Fix bug in bssplit parsing

Fix bug in bssplit parsing

I found a bug is bssplit parsing in bssplit_ddir() when trying to use
FIO on an ARM platform.  It looks like str_to_decimal() takes a "void
*" as its fourth argument, which it passes to get_multi_bytes(), which
is passes to fio_get_kb_base(), which casts it to a "struct
thread_data *".  bssplit_ddir() is passing in "struct thread_data **",
which I think is causing fio_get_kb_base() to go paw around in the
stack; I believe the different stack layouts between ARM and x86 is
causing me to see this problem.  The first change that introduces this
bug seems to be d6978a3242daad9cb7b0710b724f19225d1ed7e2.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>