igc: Refactor empty frame insertion for launch time support

Refactor the code for inserting an empty frame into a new function
igc_insert_empty_frame(). This change extracts the logic for inserting
an empty packet from igc_xmit_frame_ring() into a separate function,
allowing it to be reused in future implementations, such as the XDP
zero copy transmit function.

Remove the igc_desc_unused() checking in igc_init_tx_empty_descriptor()
because the number of descriptors needed is guaranteed.

Ensure that skb allocation and DMA mapping work for the empty frame,
before proceeding to fill in igc_tx_buffer info, context descriptor,
and data descriptor.

Rate limit the error messages for skb allocation and DMA mapping failures.

Update the comment to indicate that the 2 descriptors needed by the empty
frame are already taken into consideration in igc_xmit_frame_ring().

Handle the case where the insertion of an empty frame fails and explain
the reason behind this handling.

Signed-off-by: Song Yoong Siang <yoong.siang.song@intel.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Reviewed-by: Faizal Rahim <faizal.abdul.rahim@linux.intel.com>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://patch.msgid.link/20250216093430.957880-5-yoong.siang.song@intel.com