sys/dev/disk/dm: Add dm-delay target

sys/dev/disk/dm: Add dm-delay target

Examples
=====
  # newfs_hammer -L TEST /dev/da3 > /dev/null
  # mount_hammer /dev/da3 /HAMMER
  # hammer volume-list /HAMMER
  /dev/da3
  # cd /HAMMER
  # time git clone /usr/local/src/dragonfly
  Cloning into 'dragonfly'...
  done.
  Checking out files: 100% (34298/34298), done.
  git clone /usr/local/src/dragonfly 2.88s user 4.26s system 48% cpu 14.577 total

=====
  # newfs_hammer -L TEST /dev/da3 > /dev/null
  # kldload dm
  # dmsetup create delay1 --table '0 234436482 delay /dev/da3 0 10 /dev/da3 0 10'
  # dmsetup ls --tree
  delay1 (255:504430599)
  `- (0:504430623)
  # mount_hammer /dev/mapper/delay1 /HAMMER
  # hammer volume-list /HAMMER
  /dev/mapper/delay1
  # cd /HAMMER
  # time git clone /usr/local/src/dragonfly
  Cloning into 'dragonfly'...
  done.
  Checking out files: 100% (34298/34298), done.
  git clone /usr/local/src/dragonfly 2.78s user 3.91s system 5% cpu 2:05.05 total