netmap: enter NET_EPOCH on generic txsync

After r353292, netmap generic adapter on if_vlan interfaces panics on
asserting the NET_EPOCH. In more detail, this happens when
nm_os_generic_xmit_frame() is called, that is in the generic txsync
routine.
Fix the issue by entering the NET_EPOCH during the generic txsync.
We amortize the cost of entering/exiting over a whole batch of
transmissions.

PR: 241489
Reported by: Aleksandr Fedorov <aleksandr.fedorov@itglobal.com>