lagg: remove use of net epoch in the ioctl paths

[glebius]

Summary

Rely on LAGG_SLOCK() instead. The use of network epoch(9) here was added
in 6573d7580b851 (later tidied by 87bf9b9cbeebc) as a large sweep that
blindly substituted blocking kernel primitives with epoch(9). In these
particular code paths use of epoch(9) is incorrect and doesn't provide any
protection against a stale pointer. Recent fix 48698ead6ff0, which should
actually have removed the epoch use, created a potential sleeping in epoch
problem.