node lock when shrinking.
This is similar to r252528, applied to the above commit.
Apparently there is a race which makes necessary at least to keep the
n_size and pager size consistent when extending. Current suspect is
that iod threads perform vnode_pager_setsize() without taking the
vnode lock, which corrupts the file content.
Reported and tested by: Masachika ISHIZUKA <ish@amail.plala.or.jp>
Discussed with: rmacklem (related issues)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
6fd583583b