backports: backport skb_set_hash()

backports: backport skb_set_hash()

This backports skb_set_hash(). Two skb data structures are used
introduced at different points in time, so ifdef around that as well.
For older kernels this is a nop.

mcgrof@ergon ~/linux (git::master)$ git describe --contains bdeab99191
v3.2-rc1~129^2~458
mcgrof@ergon ~/linux (git::master)$ git describe --contains 4031ae6edb
v3.4-rc1~177^2~333^2
mcgrof@ergon ~/linux (git::master)$ git describe --contains 09323cc479
v3.14-rc1~94^2~474^2~1

Cc: Tom Herbert <therbert@google.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>