HAMMER: you can mount_hammer a UFS that was a hammer fs before

[Jan Lentfer]

If a partition contains a hammer fs and you newfs it to a UFS you can 
afterwards still mount it as hammer fs.
You can even still run hammer info and write data on the partition 
(tried with dd).

Redo the following:

atom# newfs_hammer -L pgsql /dev/ad11s2d
Volume 0 DEVICE /dev/ad11s2d    size 146.48GB
initialize freemap volume 0
---------------------------------------------
1 volume total size 146.48GB version 2
boot-area-size:       64.00MB
memory-log-size:       0.50GB
undo-buffer-size:    152.00MB
total-pre-allocated: 168.00MB
fsid:                98d1b558-c09e-11de-81f0-0122685cfb53
NOTE: Please remember that you may have to manually set up a
cron(8) job to prune and reblock the filesystem regularly.
By default, the system automatically runs 'hammer cleanup'
on a nightly basis. The periodic.conf(5) variable
'daily_clean_hammer_enable' can be unset to disable this.
Also see 'man hammer' and 'man HAMMER' for more information.
atom# newfs /dev/ad11s2d
/dev/ad11s2d: media size 149993.15MB
Warning: Block size and bytes per inode restrict cylinders per group to 89.
Warning: 1748 sector(s) in last cylinder unallocated
/dev/ad11s2d:	307185964 sectors in 74997 cylinders of 1 tracks, 4096 sectors
	149993.1MB in 843 cyl groups (89 c/g, 178.00MB/g, 22528 i/g)
super-block backups (for fsck -b #) at:
 32, 364576, 729120, 1093664, 1458208, 1822752, 2187296, 2551840, 
2916384, 3280928, 3645472, 4010016, 4374560, 4739104, 5103648, 5468192, 
5832736, 6197280, [...]

atom# mount_hammer /dev/ad11s2d /mnt

atom# mount
ROOT on / (hammer, local)
devfs on /dev (devfs, local)
/dev/serno/9SF12T4Y.s2a on /boot (ufs, local)
/pfs/@@-1:00001 on /var (null, local)
/pfs/@@-1:00002 on /tmp (null, local)
/pfs/@@-1:00003 on /usr (null, local)
/pfs/@@-1:00004 on /home (null, local)
/pfs/@@-1:00005 on /usr/obj (null, local)
/pfs/@@-1:00006 on /var/crash (null, local)
/pfs/@@-1:00007 on /var/tmp (null, local)
procfs on /proc (procfs, local)
pgsql on /mnt (hammer, local)