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.