blktrace: fix endianness in get_pdu_int()

[ Upstream commit 71df3fd82e7cccec7b749a8607a4662d9f7febdd ]

In function get_pdu_len() replace variable type from __u64 to
__be64. This fixes sparse warning.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>