filesetup: use correct random seed for non-uniform distributions

filesetup: use correct random seed for non-uniform distributions

The index in the random seed array for generating offsets is
FIO_RAND_BLOCK_OFF. So this is the index that should be used to find the
random seed when fio generates offsets following the Zipf, Pareto, and
Gaussian distributions. The previous index 4 actually corresponds to
FIO_RAND_MIX_OFF.

This change means that the default sequences of non-uniform random
offsets generated before and after this patch will differ. So users
relying on the repeatability of I/O patterns will have new repeatable
patterns after this change.

Signed-off-by: Vincent Fu <vincent.fu@samsung.com>