sysroot: Drop an fsync for origin file when writing deployments
sysroot: Drop an fsync for origin file when writing deployments

More fsync pruning.  Since we have a public API for writing the origin
file and it did a fsync before, let's preserve that.  But when writing
deployments as part of a full transaction, we rely on the global
`syncfs()`, so add an internal function for origin file writing that
doesn't.

Closes: #509
Approved by: giuseppe

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree