Ensure that these ops are properly balanced.

There are two scenarios:

1) for MMAP buf_init is called when the buffers are created and buf_cleanup
   must be called when the queue is finally freed. This scenario was always
   working.

2) for USERPTR and DMABUF it is more complicated. When a buffer is queued
   the code checks if all planes of this buffer have been acquired before.
   If that's the case, then only buf_prepare has to be called. Otherwise
   buf_cleanup needs to be called if the buffer was acquired before, then,
   once all changed planes have been (re)acquired, buf_init has to be
   called followed by buf_prepare. Should buf_prepare fail, then buf_cleanup
   must be called on the newly acquired planes to release them in.

Finally, in __vb2_queue_free we have to check if the buffer was actually
acquired before calling buf_cleanup. While that it always true for MMAP
mode, it is not necessarily true for the other modes. E.g. if you just
ca...