nd6: Do not enter net_epoch under nd6_prelist_add

[pouria]

Summary

nd6_prefix_onlink will enter net_epoch when necessary.
Also, exit net_epoch earlier in nd6_prefix_onlink,
Because we acquired a reference to ifa, and we got our ifa
from the pr->ndpr_ifp, we don't need to stay under epoch.
While here, style it.

[markj]

Seems ok.

[markj]

The commit title needs to be fixed though, "Don NOT" -> "Do not"

[zlei]

sys/netinet6/nd6_rtr.c

1441

This change looks good to me.

2145

I'd recommend against this.

A recursive NET_EPOCH_ENTER() has less performance impact vs a re-enter epoch.

CC @glebius for this change.

[pouria]

sys/netinet6/nd6_rtr.c

2145

Will wait for @glebius response

[pouria]

sys/netinet6/nd6_rtr.c

2145

Will wait for @glebius response

kind ping @glebius

[pouria]

sys/netinet6/nd6_rtr.c

2145

ping @glebius

[glebius]

sys/netinet6/nd6_rtr.c

2145

I don't understand Zhenlei's concern. We do not re-enter epoch, so there is nothing wrong with exiting earlier.