Remove nested epochs from lagg(4).

lagg_bcast_start appeared to have a bug in that was using the last
lagg port structure after exiting the epoch that was keeping that
structure alive. However, upon further inspection, the epoch was
already entered by the caller (lagg_transmit), so the epoch enter/exit
in lagg_bcast_start was actually unnecessary.

This commit generally removes uses of the net epoch via LAGG_RLOCK to
protect the list of ports when the list of ports was already protected
by an existing LAGG_RLOCK in a caller, or the LAGG_XLOCK.

It also adds a missing epoch enter/exit in lagg_snd_tag_alloc while
accessing the lagg port structures. An ifp is still accessed via an
unsafe reference after the epoch is exited, but that is true in the
current code and will be fixed in a future change.

Reviewed by: gallatin
MFC after: 1 month
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D19718