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: NIngo Molnar <mingo@elte.hu>

Update XFS for the ->splice_read/->splice_write changes.
Signed-off-by: NJens Axboe <axboe@suse.de>