io_uring/bpf_filter: pass in expected filter payload size

It's quite possible that opcodes that have payloads attached to them,
like IORING_OP_OPENAT/OPENAT2 or IORING_OP_SOCKET, that these paylods
can change over time. For example, on the openat/openat2 side, the
struct open_how argument is extensible, and could be extended in the
future to allow further arguments to be passed in.

Allow registration of a cBPF filter to give the size of the filter as
seen by userspace. If that filter is for an opcode that takes extra
payload data, allow it if the application payload expectation is the
same size than the kernels. If that is the case, the kernel supports
filtering on the payload that the application expects. If the size
differs, the behavior depends on the IO_URING_BPF_FILTER_SZ_STRICT flag:

1) If IO_URING_BPF_FILTER_SZ_STRICT is set and the size expectation
   differs, fail the attempt to load the filter.

2) If IO_URING_BPF_FILTER_SZ_STRICT isn't set, allow the filter if
   the userspace pdu size is smaller than what the kernel offers.

3) Regardless if IO_URING_BPF_FILTER_SZ_STRICT, fail loading the filter
   if the userspace pdu size is bigger than what the kernel supports.

An attempt to load a filter due to sizing will error with -EMSGSIZE.
For that error, the registration struct will have filter->pdu_size
populated with the pdu size that the kernel uses.

Reported-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>