fuse: document fuse_fsync_in.fsync_flags

The FUSE_FSYNC_DATASYNC flag was introduced by commit b6aeaded
("[PATCH] FUSE - file operations") as a magic number.  No new values have
been added to fsync_flags since.

Signed-off-by: Alan Somers <asomers@FreeBSD.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>