lib/deltas: Use fastpath for regfile writes for bare-user-only
lib/deltas: Use fastpath for regfile writes for bare-user-only

Noticed this omission while looking at the code while working on another patch.

Closes: #1270
Approved by: jlebon

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