ufs: restore uniqueness of st_dev as returned by ufs_stat()

switch ufs_stat() to use the same value for st_dev as was used by
the previous ufs_getattr() stat path.

Submitted by: gallatin
Reviewed by: mjg, imp, kib, mckusick
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D26596