Implement VOP_FDATASYNC() for msdosfs.

Standard VOP_FSYNC() implementation just syncs data buffers, and due
to this, is the correct and efficient implementation for msdosfs or
any other filesystem which uses bufer cache trivially. Provide
globally visible wrapper vop_stdfdatasync_buf() for future consumption
by other filesystems.

Reviewed by: mckusick
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D7471