io_uring/fdinfo: fix SQE_MIXED SQE displaying

io_uring/fdinfo: fix SQE_MIXED SQE displaying

When displaying pending SQEs for a MIXED ring, each 128-byte SQE
increments sq_head to skip the second slot, but the loop counter is not
adjusted. This can cause the loop to read past sq_tail by one entry for
each 128-byte SQE encountered, displaying SQEs that haven't been made
consumable yet by the application.

Match the kernel's own consumption logic in io_init_req() which
decrements what's left when consuming the extra slot.

Fixes: 1cba30bf9fdd ("io_uring: add support for IORING_SETUP_SQE_MIXED")
Signed-off-by: Jens Axboe <axboe@kernel.dk>