net: skb: Remove skb_data_area_size()

skb_data_area_size() is not needed. As Jakub pointed out [1]:
For Rx, drivers can use the size passed during skb allocation or
use skb_tailroom().
For Tx, drivers should use skb_headlen().

[1] https://lore.kernel.org/netdev/CAHNKnsTmH-rGgWi3jtyC=ktM1DW2W1VJkYoTMJV2Z_Bt498bsg@mail.gmail.com/



Signed-off-by: Ricardo Martinez <ricardo.martinez@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>