io_uring: optimise out req->opcode reloading

Looking at the assembly, the compiler decided to reload req->opcode in
io_op_defs[opcode].needs_file instead of one it had in a register, so
store it in a temp variable so it can be optimised out. Also move the
personality block later, it's better for spilling/etc. as it only
depends on @sqe, which we're keeping anyway.

By the way, zero req->opcode if it over IORING_OP_LAST, not a problem,
at the moment but is safer.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/6ba869f5f8b7b0f991c87fdf089f0abf87cbe06b.1633532552.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>