block: pass down no-wait info in the blk_plug

block: pass down no-wait info in the blk_plug

Turns out that making REQ_RAHEAD imply REQ_NOWAIT causes performance
regressions on rotational storage with few request slots. Revert the
change doing that, and instead pass down this information in the
blk_plug instead. If plug->nowait is set, then we'll set REQ_NOWAIT
on the bio when submitted.

This provides blocking read-ahead as before, while retaining the
ability to issue async buffered reads without blocking on request
shortages.

Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>