io_uring: move io_uring_get_opcode out of TP_printk

io_uring: move io_uring_get_opcode out of TP_printk

The TP_printk macro's are not supposed to use custom code ([1]) or else
tools such as perf cannot use these events.

Convert the opcode string representation to use the __string wiring that
the event framework provides ([2]).

[1]: https://lwn.net/Articles/379903/
[2]: https://lwn.net/Articles/381064/

Fixes: 033b87d24f72 ("io_uring: use the text representation of ops in trace")
Signed-off-by: Dylan Yudaken <dylany@fb.com>
Link: https://lore.kernel.org/r/20220623083743.2648321-1-dylany@fb.com
[axboe: fixup spurious removal of sq_thread assignment]
Signed-off-by: Jens Axboe <axboe@kernel.dk>