bsd.upgrade fails `umount /mnt` with a single partition disk.

[Yuichiro NAITO]

bsd.upgrade fails and shows following messages with a single partition disk.
```
Force checking of clean non-root filesystems? [no] no
umount: /mnt: Device busy
Can't umount sd0a!
```

sd0 has following partitions.

```
#                size           offset  fstype [fsize bsize   cpg]
  a:         37752672               64  4.2bsd   2048 16384 12959 # /
  b:          4176900         37752750    swap                    # none
  c:         41943040                0  unused
```

I don't want to say any thing about disk partitioning policy.
I just want to improve bsd.upgrade more stable.

This error happens in a specific environment.
In my cases, OpenBSD on FreeBSD bhyve in Intel Core i7-6700 box caused 
this error. No other environments (that I have) reproduce it.

In my investigation, the following patch caused panic in the bhyve 
environment.

diff --git a/sys/kern/vfs_subr.c b/sys/kern/vfs_subr.c
index 43a7cbd4ae9..4f4176a77ac 100644
--- a/sys/kern/vfs_subr.c
+++ b/sys/kern/vfs_subr.c
@@ -966,6 +966,7 @@ vflush_vnode(struct vnode *vp, void *arg)
                vprint("vflush: busy vnode", vp);
 #endif
        va->busy++;
+       panic("vflush_vnode: mark busy");
        return (0);
 }

I think some vnodes are still remaining while `umount /mnt`.
So, the kernel returns EBUSY.
It's right to kernel behavior and user land should handle this case.

Doing `umount /mnt` just after `fsck -p` in disrib/miniroot/install.sub.

```
3264    # Create a skeletal /etc/fstab which is usable for the upgrade process.
3265    munge_fstab
3266
3267    # fsck -p non-root filesystems in /etc/fstab.
3268    check_fs
3269
3270    # Mount filesystems in /etc/fstab.
3272    umount /mnt || { echo "Can't umount $ROOTDEV!"; exit; }
3273    mount_fs
```

Is it necessary that /mnt is mounted while `check_fs` function?
The target disk information is already read from /mnt/etc/fstab in 
`munge_fstab` function. `check_fs` uses the merged fstab in /tmp.

Just `umount /mnt` before `check_fs` can get avoid this error.
Is the following patch OK?

```
diff --git a/distrib/miniroot/install.sub b/distrib/miniroot/install.sub
old mode 100644
new mode 100755
index 16d3b6e3420..cf7e15c9754
--- a/distrib/miniroot/install.sub
+++ b/distrib/miniroot/install.sub
@@ -3243,11 +3243,12 @@ do_upgrade() {
        # Create a skeletal /etc/fstab which is usable for the upgrade process.
        munge_fstab

+       umount /mnt || { echo "Can't umount $ROOTDEV!"; exit; }
+
        # fsck -p non-root filesystems in /etc/fstab.
        check_fs

        # Mount filesystems in /etc/fstab.
-       umount /mnt || { echo "Can't umount $ROOTDEV!"; exit; }
        mount_fs

        rm -f /mnt/bsd.upgrade /mnt/auto_upgrade.conf
```

--
Yuichiro NAITO (naito.yuich...@gmail.com)