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

Since 1d238b07d5d4d9660ae0e0, krings are disabled before
a reinit cycle triggered by iflib_netmap_register.
However, this operation is actually necessary also for
any interface reinit triggered by other causes (i.e.,
ifconfig commands).
We achieve this goal by moving the krings enable/disable
operation inside iflib_stop() and iflib_init_locked().

Once here, this change also removes some redundant operations
from iflib_netmap_register(), that are already performed by
iflib_stop().

PR: 252453
MFC after: 1 week