io_uring/bpf_filter: move filter size and populate helper into struct

Rather than open-code this logic in io_uring_populate_bpf_ctx() with
a switch, move it to the issue side definitions. Outside of making this
easier to extend in the future, it's also a prep patch for using the
pdu size for a given opcode filter elsewhere.

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