DragonFlyBSD

This patch is the initial version of dm-flakey target from Linux kernel.
https://www.dragonflybsd.org/~tkusumi/diff/0001-sys-dev-disk-dm-Add-dm-flakey-target.patch

===== Example1 - Error write I/Os

1. kldload dm_target_flakey
2. dmsetup create flakey1 --table "0 234436482 flakey /dev/da3 0 1 1"
3. newfs_hammer -L TEST /dev/mapper/flakey1
   Volume 0 DEVICE /dev/mapper/flakey1 size 111.79GB
   initialize freemap volume 0
   initializing the undo map (504 MB)
   newfs_hammer: Write volume 0 (/dev/mapper/flakey1): Input/output error
4. dmsetup remove /dev/mapper/flakey1

===== Example2 - Silently drop write I/Os

1. newfs_hammer -L TEST /dev/da3 > /dev/null
2. mount_hammer /dev/da3 /HAMMER
3. cd /HAMMER
4. git clone /usr/local/src/dragonfly
   Cloning into 'dragonfly'...
   done.
   Checking out files: 100% (34434/34434), done.
5. cd
6. umount /HAMMER
7. dmsetup create flakey2 --table "0 234436482 flakey /dev/da3 0 1 1 1 drop_writes"
8. mount_hammer /dev/mapper/flakey2 /HAMMER
9. dmesg | tail -3
   HAMMER Critical error inode=-1 error=5 while flushing meta-data
   HAMMER Forcing read-only mode
   HAMMER mounted clean, no recovery needed
10. mount | grep "/HAMMER"
    TEST on /HAMMER (hammer, local, read-only)
11. cd /HAMMER/dragonfly
12. git log -p > /dev/null; echo $?
    0
13. cd
14. umount /HAMMER
15. dmsetup remove /dev/mapper/flakey2

===== Example3 - Corrupt read I/Os

1. dd if=/dev/zero of=/dev/da3 bs=1024 count=10000 >/dev/null 2>&1
2. dmsetup create flakey3 --table "0 234436482 flakey /dev/da3 0 1 1 5 corrupt_bio_byte 1 r 65 0"
3. od -tx1 /dev/mapper/flakey3 | head -10
   0000000 41 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
   0000020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 *
   0010000 41 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
   0010020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 *
   0020000 41 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
   0020020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 *
   0030000 41 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
4. dmsetup remove /dev/mapper/flakey3
5. kldunload dm_target_flakey