io_uring/kbuf: limit legacy provided buffer lists to USHRT_MAX

The buffer ID for a provided buffer is an unsigned short, and hence
there can only be 64k added to any given buffer list before having
duplicate BIDs. Cap the legacy provided buffers at 64k in the list.
This is mostly to prevent silly stall reports from syzbot, which
likes to dump tons of buffers into a list and then have kernels with
lockdep and kasan churning through them and hitting long wait times
for buffer pruning at ring exit time.

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