more code cleanups after the macro conversion:

 - standardize on 'struct pipe_inode_info *pipe' variable names
 - introduce 'pipe' temporaries to reduce mass inode->i_pipe dereferencing
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Jens Axboe <axboe@suse.de>