gianfar: Account for Tx PTP timestamp in the skb headroom

When PTP timestamping is enabled on Tx, the controller
inserts the Tx timestamp at the beginning of the frame
buffer, between SFD and the L2 frame header. This means
that the skb provided by the stack is required to have
enough headroom otherwise a new skb needs to be created
by the driver to accommodate the timestamp inserted by h/w.
Up until now the driver was relying on the second option,
using skb_realloc_headroom() to create a new skb to accommodate
PTP frames. Turns out that this method is not reliable, as
reallocation of skbs for PTP frames along with the required
overhead (skb_set_owner_w, consume_skb) is causing random
crashes in subsequent skb_*() calls, when multiple concurrent
TCP streams are run at the same time on the same device
(as seen in James' report).
Note that these crashes don't occur with a single TCP stream,
nor with multiple concurrent UDP streams, but only when multiple
TCP streams are run concurrently with the PTP packet flow
(doing skb reallocation).
This patch enforces the first method, by requesting enough
headroom from the stack to accommodate PTP frames, and so avoiding
skb_realloc_headroom() & co, and the crashes no longer occur.
There's no reason not to set needed_headroom to a large enough
value to accommodate PTP frames, so in this regard this patch
is a fix.

Reported-by: James Jurack <james.jurack@ametek.com>
Fixes: bee9e58c

 ("gianfar:don't add FCB length to hard_header_len")
Signed-off-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Link: https://lore.kernel.org/r/20201020173605.1173-1-claudiu.manoil@nxp.com


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