sys/dev/disk/dm: Fix dm-delay deps behavior for compatibility with Linux

sys/dev/disk/dm: Fix dm-delay deps behavior for compatibility with Linux

dmsetup(8)'s deps subcommand should not show two underlying
devices for both read and write if those are the same devices.
This is how Linux dm-delay behaves.

Also note that DragonFly and NetBSD's deps implementation is
inefficient in the sense that each target needs to implement
deps ioctl. dm core should be able to tell dependencies without
using target specific ioctl code. dm core in Linux kernel is
implemented this way and this is better plus less error prone.

===== using the existing code
  # kldload dm
  # dmsetup create delay1 --table '0 1000 delay /dev/da3 0 10'
  # dmsetup deps /dev/mapper/delay1
  1 dependencies  : (0, 504430623)
  # dmsetup create delay2 --table '0 1000 delay /dev/da3 0 10 /dev/da3 0 10'
  # dmsetup deps /dev/mapper/delay2
  2 dependencies  : (0, 504430623) (0, 504430623)
  # dmsetup create delay3 --table '0 1000 delay /dev/da3 0 10 /dev/da4 0 10'
  # dmsetup deps /dev/mapper/delay3
  2 dependencies  : (0, 504430623) (0, 504430631)

===== using this commit, deps for delay2 differs
  # kldload dm
  # dmsetup create delay1 --table '0 1000 delay /dev/da3 0 10'
  # dmsetup deps /dev/mapper/delay1
  1 dependencies  : (0, 504430623)
  # dmsetup create delay2 --table '0 1000 delay /dev/da3 0 10 /dev/da3 0 10'
  # dmsetup deps /dev/mapper/delay2
  1 dependencies  : (0, 504430623)
  # dmsetup create delay3 --table '0 1000 delay /dev/da3 0 10 /dev/da4 0 10'
  # dmsetup deps /dev/mapper/delay3
  2 dependencies  : (0, 504430623) (0, 504430631)