ice: make RX HW timestamp reading code more reusable

Previously, we only needed RX HW timestamp in skb path,
hence all related code was written with skb in mind.
But with the addition of XDP hints via kfuncs to the ice driver,
the same logic will be needed in .xmo_() callbacks.

Put generic process of reading RX HW timestamp from a descriptor
into a separate function.
Move skb-related code into another source file.

Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Larysa Zaremba <larysa.zaremba@intel.com>
Link: https://lore.kernel.org/r/20231205210847.28460-3-larysa.zaremba@intel.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>