Fix netmap emulated mode for FreeBSD 11 versions

[Vincenzo Maffione]

Hi,

  A change in mb_free_ext() introduced in FreeBSD 11 versions broke the
transmission support for netmap in emulated mode. This means immediate
kernel crashes for netmap users in non-native (emulated) mode.

This problem has been fixed in FreeBSD-12-CURRENT, which contains a recent
version of Netmap. However, FreeBSD 11 versions (release, stable) are still
affected, as they contain old Netmap versions.
Is it possible to push the fix (provided by me) to the relevant
release/stable branches? What is the procedure (I'm not familiar with the
FreeBSD release engineering process)?

Thanks,
  Vincenzo