factor out remove logic from if_delgroup and if_delgroups

[jacob.e.keller_intel.com]

Summary

The if_delgroups function failed to free the ifg_list structure pointer
memory. This results in a small leak on M_TEMP for each ifp group every
time an interface is detached.

This was caused because if_delgroup and if_delgroups duplicate the logic
for releasing the group memory.

Fix this by factoring out an if_freegroup function which can be called
by both if_delgroup and if_delgroups.

This results in slightly funky code, because we must drop the
IFNET_WLOCK before calling the event handler. However, we want the
IFNET_WLOCK around the group list search in if_delgroup and in the group
list iteration in if_delgroups. Further, if_delgroups must re-take the
lock after calling the new if_freegroups.

Signed-off-by: Jacob Keller <[e-mail]>

[jacob.e.keller_intel.com]

This is likely the better approach to solving the leak described in https://reviews.freebsd.org/D22215

I don't know 100% though, because the fact that the function releases the lock smells weird and like it could be confusing to look at in the future. That's why I posted both potential fixes.

[jhb]

I think the refactor is fine.

sys/net/if.c

1504

Better to add an IFNET_WLOCK_ASSERT() or some such than to just document it in a comment.

[jacob.e.keller_intel.com]

add an IFNET_WLOCK_ASSERT in if_freegroup

[erj]

Can this get regenerated? It no longer applies cleanly.

[jacob.e.keller_intel.com]

It looks like similar work was done via r355942, with slightly different (better) function names, and various style changes. This can be closed as essentially resolved/committed now.