dissect-image: make dissected_image_acquire_metadata() operate within a userns if possible

This opens the door for making the call work without privileges: if we
pass in a userns fd and DissectedImage that has mount fds then we can
acquire all information without privs.

src/dissect/dissect.c

@@ -875,7 +875,7 @@ static int action_dissect(DissectedImage *m, LoopDevice *d) {
                 fflush(stdout);
         }
 
-        r = dissected_image_acquire_metadata(m, 0);
+        r = dissected_image_acquire_metadata(m, /* userns_fd= */ -EBADF, /* extra_flags= */ 0);
         if (r == -ENXIO)
                 return log_error_errno(r, "No root partition discovered.");
         if (r == -EUCLEAN)