compat: Simplify inet_fragment backports.

compat: Simplify inet_fragment backports.

The core fragmentation handling logic is exported on all supported
kernels, so it's not necessary to backport the latest version of this.
This greatly simplifies the code due to inconsistencies between the old
per-lookup garbage collection and the newer workqueue based garbage
collection.

As a result of simplifying and removing unnecessary backport code, a few
bugs are fixed for corner cases such as when some fragments remain in
the fragment cache when openvswitch is unloaded.

Some backported ip functions need a little extra logic than what is seen
on the latest code due to this, for instance on kernels <3.17:
* Call inet_frag_evictor() before defrag
* Limit hashsize in ip{,6}_fragment logic

The pernet init/exit logic also differs a little from upstream. Upstream
ipv[46]_defrag logic initializes the various pernet fragment parameters
and its own global fragments cache. In the OVS backport, the pernet
parameters are shared while the fragments cache is separate. The
backport relies upon upstream pernet initialization to perform the
shared setup, and performs no pernet initialization of its own. When it
comes to pernet exit however, the backport must ensure that all
OVS-specific fragment state is cleared, while the shared state remains
untouched so that the regular ipv[46] logic may do its own cleanup. In
practice this means that OVS must have its own divergent implementation
of inet_frags_exit_net().

Fixes the following crash:

Call Trace:
 <IRQ>
 [<ffffffff810744f6>] ? call_timer_fn+0x36/0x100
 [<ffffffff8107548f>] run_timer_softirq+0x1ef/0x2f0
 [<ffffffff8106cccc>] __do_softirq+0xec/0x2c0
 [<ffffffff8106d215>] irq_exit+0x105/0x110
 [<ffffffff81737095>] smp_apic_timer_interrupt+0x45/0x60
 [<ffffffff81735a1d>] apic_timer_interrupt+0x6d/0x80
 <EOI>
 [<ffffffff8104f596>] ? native_safe_halt+0x6/0x10
 [<ffffffff8101cb2f>] default_idle+0x1f/0xc0
 [<ffffffff8101d406>] arch_cpu_idle+0x26/0x30
 [<ffffffff810bf3a5>] cpu_startup_entry+0xc5/0x290
 [<ffffffff810415ed>] start_secondary+0x21d/0x2d0
Code:  Bad RIP value.
RIP  [<ffffffffa0177480>] 0xffffffffa0177480
 RSP <ffff88003f703e78>
CR2: ffffffffa0177480
---[ end trace eb98ca80ba07bd9c ]---
Kernel panic - not syncing: Fatal exception in interrupt

Signed-off-by: Joe Stringer <joe@ovn.org>
Acked-by: Pravin B Shelar <pshelar@ovn.org>