thinktime: Fix missing re-init thinktime when using ramptime

thinktime: Fix missing re-init thinktime when using ramptime

Prevent I/O bursts after ramptime due to thinktime.

Each thread generates a certain amount of I/O requests,
configured by thinktime_blocks.
When ramptime ends, thinktime_blocks can't control I/O.
Because thinktime_blocks are not reinitialized after ramptime.
I fixed it by reinitializing last_thinktime and last_thinktime_blocks
when ramptime ended.

Signed-off by: Suho Son <suho.son@samsung.com>