inet_frag: Limit ABI change in 4.9.51

inet_frag: Limit ABI change in 4.9.51

Commit fb452a1aa3fd 'Revert "net: use lib/percpu_counter API for
fragmentation mem accounting"' changed the type of inet_frag::mem
from struct percpu_counter to atomic_t.  There are few modules
that actually depend on this, but struct inet_frag is embedded in
various other structures that end up affecting symbol versions
for a huge number of networking APIs.

So add padding to keep the structure size unchanged, hide the type
change from genksyms, and rename a key inet_frag function so that
those modules that actually depend on the type of inet_frag::mem do
see an ABI change.

Gbp-Pq: Topic debian
Gbp-Pq: Name inet_frag-limit-abi-change-in-4.9.51.patch