udev: fix bounds check in dev_if_packed_info()

udev: fix bounds check in dev_if_packed_info()

The check compared bLength against (size - sizeof(descriptor)), which
is an absolute limit unrelated to the current buffer position. Since
bLength is uint8_t (max 255), this can never exceed size - 9 for any
realistic input, making the check dead code.

Use (size - pos) instead so the check actually catches descriptors
that extend past the end of the read data.

Fixes: https://github.com/systemd/systemd/issues/41570