pipe: stop using ->can_merge

pipe: stop using ->can_merge

Al Viro pointed out that since there is only one pipe buffer type to which
new data can be appended, it isn't necessary to have a ->can_merge field in
struct pipe_buf_operations, we can just check for a magic type.

Suggested-by: Al Viro <[email protected]>
Signed-off-by: Jann Horn <[email protected]>
Signed-off-by: Al Viro <[email protected]>