sys/dev/disk/dm: Remove dm/targets/ directory and move its entries to dm/

Device mapper targets' path
sys/dev/disk/dm/targets/<name>/dm_target_<name>.c
are too deep for no reason.

This commit remov

sys/dev/disk/dm: Remove dm/targets/ directory and move its entries to dm/

Device mapper targets' path
sys/dev/disk/dm/targets/<name>/dm_target_<name>.c
are too deep for no reason.

This commit removes targets/ directory which isn't necessary at all,
assuming userspace has no dependencies on this (and it shouldn't either).

Neither NetBSD (sys/dev/dm/dm_target_*.c) nor Linux kernel (drivers/md/dm*.c)
has a directory tree like this, which is better.

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