io_uring: add type to op enum

It is useful to have a type enum for opcodes, to allow the compiler to
assert that every value is used in a switch statement.

Signed-off-by: Dylan Yudaken <dylany@fb.com>
Link: https://lore.kernel.org/r/20220426082907.3600028-2-dylany@fb.com


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