ublk: don't cast registered buffer index to int

ublk: don't cast registered buffer index to int

io_buffer_register_bvec() takes index as an unsigned int argument, but
ublk_register_io_buf() casts ub_cmd->addr (a u64) to int. Remove the
misleading cast and instead pass index as an unsigned value to
ublk_register_io_buf() and ublk_unregister_io_buf().

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250301190317.950208-1-csander@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>