ANDROID: fixup task_struct to avoid ABI breakage

Reuse task_struct.worker_private to store task_dma_buf_info pointer and
avoid adding new task_struct members that would lead to ABI breakage.
For kthreads we add task_dma_buf_info member to struct kthread to store
dmabuf accounting information. io_workers do not map dmabufs, so we
do not store dmabuf accounting information for them.

[surenb: backport from 6.6 requires additional reset of worker_private
in io_worker_exit() to avoid double-freeing]
Bug: 424648392
Change-Id: I90bf6762db62f36a2ef80e345bfa811016cdff8b
Signed-off-by: Suren Baghdasaryan <surenb@google.com>