Enable soft updates by default for UFS2 filesystems.

Differential Revision: https://reviews.freebsd.org/D45201

(cherry picked from commit 61dece6d27fb2436928ca93d65667b358e05aa7b)
(cherry picked from commit 7465f9fc06467e57d3bbf2430031fe51d8182c30)

Approved by: re (Colin Percival)