net: fix comment above build_skb()

build_skb() specifies that the data parameter must come from a kmalloc'd
area, this is only true if frag_size equals 0, because then build_skb()
will use kzsize(data) to figure out the actual data size. Update the
comment to reflect that special condition.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -309,7 +309,8 @@ EXPORT_SYMBOL(__alloc_skb);
  * @frag_size: size of fragment, or 0 if head was kmalloced
  *
  * Allocate a new &sk_buff. Caller provides space holding head and
- * skb_shared_info. @data must have been allocated by kmalloc()
+ * skb_shared_info. @data must have been allocated by kmalloc() only if
+ * @frag_size is 0, otherwise data should come from the page allocator.
  * The return is the new skb buffer.
  * On a failure the return is %NULL, and @data is not freed.
  * Notes :