dummynet: fix ip_dn_vnet_init() / dummynet_task() race

[kp]

Summary

If dummynet_task() is run on a vnet where dummynet is still initialising
(i.e. still running ip_dn_vnet_init()) we can attempt to use an
uninitialised mutex.

We can use the existing init_done field to check if the per-vnet
V_dn_cfg is fully set up, if we ensure that it's only set to 1 when
we've done all of the init work.

Reported by: Alfredo Dal'Ava Júnior <[e-mail]>
Sponsored by: Rubicon Communications, LLC ("Netgate")

[jkim]

This commit breaks non-VIMAGE kernel build. Try MINIMAL kernel for example.

/usr/src/sys/netpfil/ipfw/ip_dn_io.c:674:4: error: 'continue' statement not in loop statement
                        continue;
                        ^
1 error generated.

[kp]

In D35716#810455, @jkim wrote:

This commit breaks non-VIMAGE kernel build. Try MINIMAL kernel for example.

/usr/src/sys/netpfil/ipfw/ip_dn_io.c:674:4: error: 'continue' statement not in loop statement
                        continue;
                        ^
1 error generated.

I'm aware and am testing a fix (to the vnet macros).