RDMA/hns: Adjust lp_pktn_ini dynamically

lp_pktn_ini means the number of loopback slice packets for long messages,
it should depend on MTU(fixed to 4096B currently) and max size of SQ
inline.

Link: https://lore.kernel.org/r/1588242691-12913-3-git-send-email-liweihang@huawei.com
Signed-off-by: Weihang Li <liweihang@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>