dpaa2: Avoid dpaa2_cmd race conditions

[dsl]

Summary

struct dpaa2_cmd is no longer malloc'ed, but can be allocated on stack
and initialized with DPAA2_CMD_INIT() on demand. Drivers stopped caching
their DPAA2 command objects (and associated tokens) in the software
contexts in order to avoid using them concurrently.

Details: https://github.com/mcusim/freebsd-src/issues/14

[dsl]

diff has been updated to provide full context.

[bz]

I had some simplifications when I merged your earlier work but merging those in into this would be complicated. I'll leave that to "code cleanup" at some point. Thanks a lot for doing the work!