b_vflags update requries bufobj lock

The trunc_dependencies() issue was reported by Alexander Lochmann
<alexander.lochmann@tu-dortmund.de>, who found the problem by performing
lock analysis using LockDoc, see https://doi.org/10.1145/3302424.3303948.

Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week