Fix rtsold(8) remote buffer overflow vulnerability. [SA-14:20]

Fix memory leak in sandboxed namei lookup. [SA-14:22]
MFC r273402:
  Fix an issue where a FreeBSD virtual machine provisioned in
  the Microsoft Azure service does not recognize the second
  attached disk on the system.

PR:		194376
Insta-MFC OK:	re (rodrigc)
Sponsored by:	The FreeBSD Foundation
Bump __FreeBSD_version to track SA-14:20, SA-14:21, SA-14:22,
SA-14:23

Approved by:	re (implicit)
Sponsored by:	The FreeBSD Foundation