netmap: restore hwofs and support it in iflib

Restore the hwofs functionality temporarily disabled by
7ba6ecf216fb15e8b147db2 to prevent issues with iflib.
This patch brings the necessary changes to iflib to
enable howfs to allow interface restarts without
disrupting netmap applications actively using its
rings.
After this change, it becomes possible for multiple
non-cooperating netmap applications to use non-overlapping
subsets of the available netmap rings without clashing
with each other.

PR: 252453
MFC after: 1 week