ixgbe: fix crash in build_skb Rx code path

ixgbe: fix crash in build_skb Rx code path

Add check for build_skb enabled ring in ixgbe_dma_sync_frag().
In that case &skb_shinfo(skb)->frags[0] may not always be set which
can lead to a crash. Instead we derive the page offset from skb->data.

Fixes: 42073d91a214
("ixgbe: Have the CPU take ownership of the buffers sooner")
CC: stable <[email protected]>
Reported-by: Ambarish Soman <[email protected]>
Suggested-by: Alexander Duyck <[email protected]>
Signed-off-by: Emil Tantilov <[email protected]>
Tested-by: Andrew Bowers <[email protected]>
Signed-off-by: Jeff Kirsher <[email protected]>
Signed-off-by: David S. Miller <[email protected]>