net: fix documentation of skb_needs_linearize().

skb_needs_linearize() does not check highmem DMA as it does not call
illegal_highdma() anymore, so there is no need to mention highmem DMA here.

(Indeed, ~NETIF_F_SG flag, which is checked in skb_needs_linearize(), can
be set when illegal_highdma() returns true, and we are assured that
illegal_highdma() is invoked prior to skb_needs_linearize() as
skb_needs_linearize() is a static method called only once.
But ~NETIF_F_SG can be set not only there in this same invocation path.
It can also be set when can_checksum_protocol() returns false).

see commit 02932ce9,
Convert skb_need_linearize() to use precomputed features.
Signed-off-by: Rami Rosen <rosenr@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -2184,9 +2184,7 @@ EXPORT_SYMBOL(netif_skb_features);
 /*
  * Returns true if either:
  *	1. skb has frag_list and the device doesn't support FRAGLIST, or
- *	2. skb is fragmented and the device does not support SG, or if
- *	   at least one of fragments is in highmem and device does not
- *	   support DMA from it.
+ *	2. skb is fragmented and the device does not support SG.
  */
 static inline int skb_needs_linearize(struct sk_buff *skb,
 				      int features)