io_uring: optimise non-drain path

Replace drain checks with one-way flag set upon seeing the first
IOSQE_IO_DRAIN request. There are several places where it cuts cycles
well:

1) It's much faster than the fast check with two
conditions in io_drain_req() including pretty complex
list_empty_careful().

2) We can mark io_queue_sqe() inline now, that's a huge win.

3) It replaces timeout and drain checks in io_commit_cqring() with a
single flags test. Also great not touching ->defer_list there without a
reason so limiting cache bouncing.

It adds a small amount of overhead to drain path, but it's negligible.
The main nuisance is that once it meets any DRAIN request in io_uring
instance lifetime it will _always_ go through a slower path, so
drain-less and offset-mode timeout less applications are preferable.
The overhead in that case would be not big, but it's worth to bear in
mind.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/98d2fff8c4da5144bb0d08499f591d4768128ea3.1623709150.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

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