backports: backport definition of struct frag_queue

backports: backport definition of struct frag_queue

Commit b836c99f by Amerigo which got added on v3.7 generalized
the conntrack struct frag_queue while unifying the conntrack
reassembly expire code with a standard one. Although we won't
use the expiry code, another subsystem which we backport, the
ieee802154 6lowpan subsystem, makes use of this structure for
its own reassembly code. This makes that structure available
for older kernels.

mcgrof@ergon ~/linux-next (git::master)$ git describe --contains b836c99f
v3.7-rc1~145^2~136

Cc: Amerigo Wang <amwang@redhat.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>