Merge r261590, r261592 from head:
Remove identical vnet sysctl handlers, and handle CTLFLAG_VNET
in the sysctl_root().

Note: SYSCTL_VNET_* macros can be removed as well. All is
needed to virtualize a sysctl oid is set CTLFLAG_VNET on it.
But for now keep macros in place to avoid large code churn.

MFC r258675: Fix build.

MFC r258622: dtrace sdt: remove the ugly sname parameter of SDT_PROBE_DEFINE

Merge r261590, r261592 from head:
Remove identical vnet sysctl handlers, and handle CTLFLAG_VNET
in the sysctl_root().

Note: SYSCTL_VNET_* macros can be removed as well. All is
needed to virtualize a sysctl oid is set CTLFLAG_VNET on it.
But for now keep macros in place to avoid large code churn.

MFC r258675: Fix build.

MFC r258622: dtrace sdt: remove the ugly sname parameter of SDT_PROBE_DEFINE