ifnet: add if_foreach_sleep() to allow ifnet iterations with sleep.

[melifaro]

Summary

Some kernel customers exporting network interfaces may require sleep during the iteration. Netlink and linsysfs can be the examples.

This change adds the generic iteration interface, similar to if_foreach(), but allowing the callback to sleep.

[melifaro]

Update match function signature.

[dchagin]

sys/net/if.c

4591

seems should be error = cb();

[melifaro]

Fix error handling.

[dchagin]

https://reviews.freebsd.org/D38901
Excelent to me

[rpokala]

As discussed in email, if_foreach_sleep() sounds like the loop itself inserts a delay. I suggest changing it to if_foreach_sleepable(), which more accurately conveys that it can sleep (or use callbacks that can sleep), not that that it does sleep.