skb: Add skb_head_is_locked helper function

This patch adds support for a skb_head_is_locked helper function.  It is
meant to be used any time we are considering transferring the head from
skb->head to a paged frag.  If the head is locked it means we cannot remove
the head from the skb so it must be copied or we must take the skb as a
whole.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 84e69e0..7b2d351 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -4568,7 +4568,7 @@
 		    skb_shinfo(from)->nr_frags >= MAX_SKB_FRAGS)
 			return false;
 
-		if (!from->head_frag || skb_cloned(from))
+		if (skb_head_is_locked(from))
 			return false;
 
 		delta = from->truesize - SKB_DATA_ALIGN(sizeof(struct sk_buff));