net: dsa: implement a central TX reallocation procedure

At the moment, taggers are left with the task of ensuring that the skb
headers are writable (which they aren't, if the frames were cloned for
TX timestamping, for flooding by the bridge, etc), and that there is
enough space in the skb data area for the DSA tag to be pushed.

Moreover, the life of tail taggers is even harder, because they need to
ensure that short frames have enough padding, a problem that normal
taggers don't have.

The principle of the DSA framework is that everything except for the
most intimate hardware specifics (like in this case, the actual packing
of the DSA tag bits) should be done inside the core, to avoid having
code paths that are very rarely tested.

So provide a TX reallocation procedure that should cover the known needs
of DSA today.

Note that this patch also gives the network stack a good hint about the
headroom/tailroom it's going to need. Up till now it wasn't doing that.
So the reallocation procedure should really be there only for the
exceptional cases, and for cloned packets which need to be unshared.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Christian Eggers <ceggers@arri.de> # For tail taggers only
Tested-by: Kurt Kanzenbach <kurt@linutronix.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>