nvme: Move bools to fill hole

The two bools in nvme_request create a 6 byte hole today. Move them to
after retires to fill the 4 byte hole there and add a spare[2] to make
nvme_request 8 bytes smaller. spare[2] isn't strictly necessary, but
documents how many bytes we have left in that hole, as the number of
booleans will increase shortly.

Suggested by: chuck
Sponsored by: Netflix