gianfar: Replace skb_realloc_headroom with skb_cow_head for PTP

gianfar: Replace skb_realloc_headroom with skb_cow_head for PTP

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 skb_realloc_headroom()
to create new skbs to accommodate PTP frames.  Turns out that
this method is not reliable in this context at least, as
skb_realloc_headroom() for PTP frames can cause random crashes,
mostly in subsequent skb_*() calls, when multiple concurrent
TCP streams are run at the same time with the PTP flow
on the same device (as seen in James' report).  I also noticed
that when the system is loaded by sending multiple TCP streams,
the driver receives cloned skbs in large numbers.
skb_cow_head() instead proves to be stable in this scenario,
and not only handles cloned skbs too but it's also more efficient
and widely used in other drivers.
The commit introducing skb_realloc_headroom in the driver
goes back to 2009, commit 93c1285c5d92
("gianfar: reallocate skb when headroom is not enough for fcb").
For practical purposes I'm referencing a newer commit (from 2012)
that brings the code to its current structure (and fixes the PTP
case).

Fixes: 9c4886e5e63b ("gianfar: Fix invalid TX frames returned on error queue when time stamping")
Reported-by: James Jurack <james.jurack@ametek.com>
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Link: https://lore.kernel.org/r/20201029081057.8506-1-claudiu.manoil@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>