The old thread priority must be stored as part of the EPOCH(9) tracker.

Else recursive use of EPOCH(9) may cause the wrong priority to be restored.

Bump the __FreeBSD_version due to changing the thread and epoch tracker
structure.

Differential Revision: https://reviews.freebsd.org/D30375
Reviewed by: markj@
MFC after: 1 week
Sponsored by: Mellanox Technologies // NVIDIA Networking