Partially revert r255986: do not call VOP_FSYNC() when helping
bufdaemon in getnewbuf(), do use buf_flush(). The difference is that
bufdaemon uses TRYLOCK to get buffer locks, which allows calls to
getnewbuf() while another buffer is locked.

Reported and tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week