[kernel] Use kernel proc space as kernel space

As mentioned in the last commit, with processes owning spaces, there was
a weird extra space in the "kernel" process that owns the kernel
threads. Now we use that space as the global kernel space, and don't
create a separate one.

src/kernel/buffer_cache.cpp

@@ -5,8 +5,6 @@
 #include "page_manager.h"
 #include "vm_space.h"
 
-extern vm_space g_kernel_space;
-
 using memory::frame_size;
 using memory::kernel_stack_pages;
 using memory::kernel_buffer_pages;