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).

src/tmpfiles/tmpfiles.c

@@ -856,7 +856,7 @@ static int path_open_parent_safe(const char *path) {
         if (!dn)
                 return log_oom();
 
-        fd = chase_symlinks(dn, NULL, CHASE_OPEN|CHASE_SAFE|CHASE_WARN, NULL);
+        fd = chase_symlinks(dn, arg_root, CHASE_OPEN|CHASE_SAFE|CHASE_WARN, NULL);
         if (fd < 0 && fd != -ENOLINK)
                 return log_error_errno(fd, "Failed to validate path %s: %m", path);
 
@@ -877,7 +877,7 @@ static int path_open_safe(const char *path) {
                                        "Failed to open invalid path '%s'.",
                                        path);
 
-        fd = chase_symlinks(path, NULL, CHASE_OPEN|CHASE_SAFE|CHASE_WARN|CHASE_NOFOLLOW, NULL);
+        fd = chase_symlinks(path, arg_root, CHASE_OPEN|CHASE_SAFE|CHASE_WARN|CHASE_NOFOLLOW, NULL);
         if (fd < 0 && fd != -ENOLINK)
                 return log_error_errno(fd, "Failed to validate path %s: %m", path);
 
@@ -2256,7 +2256,7 @@ static int process_item(Item *i, OperationMask operation) {
 
         i->done |= operation;
 
-        r = chase_symlinks(i->path, NULL, CHASE_NO_AUTOFS|CHASE_WARN, NULL);
+        r = chase_symlinks(i->path, arg_root, CHASE_NO_AUTOFS|CHASE_WARN, NULL);
         if (r == -EREMOTE) {
                 log_notice_errno(r, "Skipping %s", i->path);
                 return 0;