net: mhi-net: Add re-aggregation of fragmented packets

When device side MTU is larger than host side MTU, the packets
(typically rmnet packets) are split over multiple MHI transfers.
In that case, fragments must be re-aggregated to recover the packet
before forwarding to upper layer.

A fragmented packet result in -EOVERFLOW MHI transaction status for
each of its fragments, except the final one. Such transfer was
previously considered as error and fragments were simply dropped.

This change adds re-aggregation mechanism using skb chaining, via
skb frag_list.

A warning (once) is printed since this behavior usually comes from
a misconfiguration of the device (e.g. modem MTU).

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Acked-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Link: https://lore.kernel.org/r/1612428002-12333-1-git-send-email-loic.poulain@linaro.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>