msdosfs: Fix a leak of dirent padding bytes
This was missed in r340856 / commit
6d2e2df764199f0a15fd743e79599391959cc17d. Three bytes from the kernel
stack may be leaked when reading directory entries.
Reported by: Syed Faraz Abrar <faraz@elttam.com>
MFC after: 3 days
Sponsored by: The FreeBSD Foundation