io_uring: add io_uring_get_opcode

In some debug scenarios it is useful to have the text representation of
the opcode. Add this function in preparation.

Signed-off-by: Dylan Yudaken <dylany@fb.com>
Link: https://lore.kernel.org/r/20220426082907.3600028-3-dylany@fb.com


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