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>
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 599f90446376370eb365a0fde857ea2b5766873a)
(cherry picked from commit 6d0a2f9d2ffce3d94c9a523d7779f791355d3677)

Approved by: so
Security: CVE-2020-25579