blkcg: implement BPF_PROG_TYPE_IO_COST

Currently, blkcg implements one builtin IO cost model - lienar.  To
allow customization and experimentation, allow a bpf program to
override IO cost model.

v2: bash completion and documentation updates suggested by Quentin.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Quentin Monnet <quentin.monnet@netronome.com>
16 files changed