This corrects an issue introduced in b4cb3fe0e3, where a freshly
allocated `DOS_FS` structure would not be initialized properly before
use in `dos_open`.
In case of FAT32 file systems, this would leave `fs->dirents`
uninitialized and - depending on its content and due to checks in
`parsebs` - prevent mounting the file system successfully.
This particularily impacted the EFI loader, as it was sometimes not
able to read files from a FAT32-formatted EFI partition, including
LoaderEnv (`/efi/freebsd/loader.env`).
Accepted by: imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D36482
(cherry picked from commit 0eb736c0f6)
6d162b90b5