ieee802154: add generic header handling routines

Add a generic set of 802.15.4 header operations on sk_buffs: push header
onto skb, pull header from skb, and peek address fields from the mac_hdr
part of an skb.

These routines support the full 802.15.4 header as described in the
standard, including the security header. They are useful for everything
that must create, modify or read 802.15.4 headers, which of course
includes the wpan rx/tx path, but also 6lowpan. In the future,
link-layer security will also require means to modify packet headers.

Signed-off-by: Phoebe Buckheister <phoebe.buckheister@itwm.fraunhofer.de>
mac802154: use new header ops in wpan devices

Replace the old header creation/parsing with the new header operations.
This reduces code duplication that existed between
mac802154_parse_frame_start and mac802154_header_parse. This also fixes
a bug that caused 802.15.4 frames with link layer security enabled to be
misparsed, leading to parts of the header being passed to upper layers
as data.

Signed-off-by: Phoebe Buckheister <phoebe.buckheister@itwm.fraunhofer.de>
ieee802154: remove addresses from mac_cb

The mac802154 stack itself does not strictly require these fields: the
tx path never even touches them, and this patch modifies the rx path to
explicitly carry a parsed header.

One notable user of these fields was 6lowpan, which accessed them after
the skb had been passed to it through dev_queue_xmit. 6lowpan was
changed to peek the addresses from a given skb.

Signed-off-by: Phoebe Buckheister <phoebe.buckheister@itwm.fraunhofer.de>
ieee802154: remove seq member of mac_cb

The seq member is only used to initialize the sequence number field of
the 802.15.4 header. This field has relevance only for low-level
functionality like frame acknowledgement and is of no importance to
upper layers. Upper layers should not be allowed to set this field at
all.

Signed-off-by: Phoebe Buckheister <phoebe.buckheister@itwm.fraunhofer.de>