pipe: merge anon_pipe_buf*_ops

All the op vectors are exactly the same, they are just used to encode
packet or nomerge behavior.  There already is a flag for the packet
behavior, so just add a new one to allow for merging.  Inverting it vs
the previous nomerge special casing actually allows for much nicer code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>