ofed: mask seq_num identifier to occupy only 3 bytes

The seq_num among other things is used to assign rq_psn value, which is
a 24-bit identifier. When the seq_num is full 4-byte value, we are
usually receiving: '_ib_modify_qp rq_psn overflow, masking to 24 bits'
warning.

This is burdensome for running rdma traffic with large number of
connections, because the number of logs is growing fast.

Signed-off-by: Bartosz Sobczak <bartosz.sobczak@intel.com>
Signed-off-by: Eric Joyner <erj@FreeBSD.org>

Reviewed by: kib@, erj@
MFC after: 3 days
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D41531