netmap: vtnet: stop krings during interface reset

Similarly to what done for iflib in 1d238b07d5d4d9660ae0e,
this patch prevents access to the krings during the interface
reset triggered by netmap_register().

MFC after: 1 week

(cherry picked from commit 9ac59d42c0b4b6cd9c36a5dace7f49753c2e175a)

netmap: vtnet: enable/disable krings on any interface reinit

See 3d65fd97e85ab807f3b for a detailed explanation.

PR: 252453
MFC after: 1 week

(cherry picked from commit bb714db6d39583a9fbf5d11849c5e2365e7c0d80)

netmap: vtnet: fix RX initialization after netmap_reset()

At device reset, we must not publish those netmap receive buffers
that are owned by userspace (nm_kr_rxspace).

MFC after: 1 week

(cherry picked from commit 3005e10ddbfbec3ecf46a080607bb0d85986eee5)