net: add __pskb_copy_fclone and pskb_copy_for_clone

There are several instances where a pskb_copy or __pskb_copy is
immediately followed by an skb_clone.

Add a couple of new functions to allow the copy skb to be allocated
from the fclone cache and thus speed up subsequent skb_clone calls.

Reviewed-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
[backport from net-next]
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
mptcp: remove mptcp_pskb_copy

Now that we have pskb_copy_for_clone() we can use it and we can remove
mptcp_pskb_copy.

Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>