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 adding new member to the epoch tracker
structure.

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

(cherry picked from commit ef0f7ae934b04a4f90e051d701ba539dd04e7d5b)