Fix :<nr> suffix with random read/write causing 0 initial offset

Fix :<nr> suffix with random read/write causing 0 initial offset

When using the :<nr> suffix with random reads or writes, the initial
offset would be set to 0 for the first nr-1 operations. This happened
because td->ddir_seq_nr was initialized to the specified option value,
when it needs to always be initialized to 1, so that the first call to
get_next_offset leads to choosing a new random offset for the first nr
operations.

Signed-off-by: Nick Neumann nick@pcpartpicker.com