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.
This aliasing works because task_struct.worker_private is used only for
kthreads and io_workers which task_dma_buf_info is used for user tasks.

Bug: 424648392
Change-Id: I2caa708d8a729095b308932c1b35c3157835639b
Signed-off-by: Suren Baghdasaryan <surenb@google.com>