msdosfs: Fix a leak of dirent padding bytes

This was missed in r340856 / commit
6d2e2df764.  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

sys/fs/msdosfs/msdosfs_vnops.c

@@ -1684,6 +1684,7 @@ msdosfs_readdir(struct vop_readdir_args *ap)
 			dirbuf.d_reclen = GENERIC_DIRSIZ(&dirbuf);
 			/* NOTE: d_off is the offset of the *next* entry. */
 			dirbuf.d_off = offset + sizeof(struct direntry);
+			dirent_terminate(&dirbuf);
 			if (uio->uio_resid < dirbuf.d_reclen) {
 				brelse(bp);
 				goto out;