Add softrandommap and --alloc-size options

Add softrandommap and --alloc-size options

The softrandommap job option allows fio to continue if it cannot
allocate from the smalloc backing for the random map. Or the user
can specify --alloc-size with a size larger than the default of 1024k
to increase the size of the shared allocation pool.

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