Despite the call to vtruncbuf() from detrunc(), which results in
zeroing part of the partial page after EOF, there still is a
possibility to retain the stale data which is revived on file
enlargement. If the filesystem block size is greater than the page
size, partial block might keep other after-EOF pages wired and they
get reused then. Fix it by zeroing whole part of the partial buffer
after EOF, not relying on vnode_pager_setsize().
PR: 236977
Reported by: asomers
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
c973ee9e06