io_uring: use the text representation of ops in trace

It is annoying to translate opcodes to textwhen tracing io_uring. Use the
io_uring_get_opcode function instead to use the text representation.

A downside here might have been that if the opcode is invalid it will not
be obvious, however the opcode is already overridden in these cases to
0 (NOP) in io_init_req(). Therefore this is a non issue.

Signed-off-by: Dylan Yudaken <dylany@fb.com>
Link: https://lore.kernel.org/r/20220426082907.3600028-5-dylany@fb.com
[axboe: don't include register, those are not req opcodes]
Signed-off-by: Jens Axboe <axboe@kernel.dk>