dpaa2: Avoid dpaa2_cmd race conditions

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.

Reviewed by: bz
Approved by: bz (mentor)
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D39509