[PATCH] introduce a "kernel-internal pipe object" abstraction

separate out the 'internal pipe object' abstraction, and make it
usable to splice. This cleans up and fixes several aspects of the
internal splice APIs and the pipe code:

 - pipes: the allocation and freeing of pipe_inode_info is now more symmetric
   and more streamlined with existing kernel practices.

 - splice: small micro-optimization: less pointer dereferencing in splice
   methods

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Update XFS for the ->splice_read/->splice_write changes.

Signed-off-by: Jens Axboe <axboe@suse.de>