bio_associate_current — 
     associate a bio with current
 
| int bio_associate_current ( | struct bio * bio ); | 
   Associate bio with current if it hasn't been associated yet.  Block
   layer will treat bio as if it were issued by current no matter which
   task actually issues it.
   
   This function takes an extra reference of task's io_context and blkcg
   which will be put when bio is released.  The caller must own bio,
   ensure current->io_context exists, and is responsible for synchronizing
   calls to this function.