net: qmi_wwan: Add pass through mode

net: qmi_wwan: Add pass through mode

Pass through mode is to allow packets in MAP format to be passed
on to the stack. rmnet driver can be used to process and demultiplex
these packets.

Pass through mode can be enabled when the device is in raw ip mode only.
Conversely, raw ip mode cannot be disabled when pass through mode is
enabled.

Userspace can use pass through mode in conjunction with rmnet driver
through the following steps-

1. Enable raw ip mode on qmi_wwan device
2. Enable pass through mode on qmi_wwan device
3. Create a rmnet device with qmi_wwan device as real device using netlink

Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
Acked-by: Bjørn Mork <bjorn@mork.no>
Link: https://lore.kernel.org/r/1611560015-20034-1-git-send-email-subashab@codeaurora.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>