io_uring/kbuf: remove obsolete buf_nr_pages and update comments

io_uring/kbuf: remove obsolete buf_nr_pages and update comments

The buf_nr_pages field in io_buffer_list was previously used to
determine whether the buffer list uses ring-provided buffers or classic
provided buffers. This is now determined by checking the IOBL_BUF_RING
flag.

Remove the buf_nr_pages field and update related comments.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>