dm_target_crypt - use per-instance mpipes, KTR

dm_target_crypt - use per-instance mpipes, KTR

 * Add KTR support (KTR_DMCRYPT) to monitor the read and write strategy
   paths.

 * Make the mpipes for reads and writes per-instance instead of for all
   targets. This solves the issue where two instances are interdependent
   and one starves the other, which will cause a deadlock. This way each
   instance is guaranteed to have some buffers.

 * Change the maximum size of the mpipes from 0.5% of physical memory to
   0.2%, as this limit is now per-instance and an average of 2 or 3
   crypt targets sounds reasonable.