null_blk: change type of completion_nsec to unsigned long

This commit at least doubles the maximum value for
completion_nsec. This helps in special cases where one wants/needs to
emulate an extremely slow I/O (for example to spot bugs).

Signed-off-by: Paolo Valente <paolo.valente@unimore.it>
Signed-off-by: Arianna Avanzini <avanzini@google.com>
Signed-off-by: Jens Axboe <axboe@fb.com>