tmpfiles: pass arg_root to chase_symlinks as the root prefix

tmpfiles: pass arg_root to chase_symlinks as the root prefix

This informs chase_symlinks that symlinks should be treated as if
the path given by --root= is the root of their file system.

With the parent commit, this allows tmpfiles to create files as the
root user under a prefix that may be owned by an unprivileged user.
In particular, this fixes the case where tmpfiles generates initial
files in a staging root directory for packaging under a directory
owned by the unprivileged packager user (e.g. in Gentoo).