It is only used to allocate the kernel map, and the exec and pipe
submaps. Rename kmem_suballoc() to kmem_subinit(), and remove
vm_map_create().
Rename the pipe_map field of struct pipe to avoid conflicting with the
macro. I think pipe_pages is a better name since the structure really
only tracks the wired sources pages used during a direct write, not a
mapping itself.