mpls: Reduce skb re-allocations due to skb_cow()

mpls_xmit() needs to prepend the MPLS-labels to the packet. That implies
one needs to make sure there is enough space for it in the headers.

Calling skb_cow() implies however that one wants to change even the
playload part of the packet (which is not true for MPLS). Thus, call
skb_cow_head() instead, which is what other tunnelling protocols do.

Running a server with this comm it entirely removed the calls to
pskb_expand_head() from the callstack in mpls_xmit() thus having
significant CPU-reduction, especially at peak times.

Cc: Roopa Prabhu <roopa@nvidia.com>
Reported-by: Craig Taylor <cmtaylor@apple.com>
Signed-off-by: Christoph Paasch <cpaasch@apple.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240815161201.22021-1-cpaasch@apple.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/mpls/mpls_iptunnel.c

@@ -106,7 +106,7 @@ static int mpls_xmit(struct sk_buff *skb)
 		hh_len = 0;
 
 	/* Ensure there is enough space for the headers in the skb */
-	if (skb_cow(skb, hh_len + new_header_size))
+	if (skb_cow_head(skb, hh_len + new_header_size))
 		goto drop;
 
 	skb_set_inner_protocol(skb, skb->protocol);