x86: introduce page pool in cpa

DEBUG_PAGEALLOC was not possible on 64-bit due to its early-bootup
hardcoded reliance on PSE pages, and the unrobustness of the runtime
splitup of large pages. The splitup ended in recursive calls to
alloc_pages() when a page for a pte split was requested.

Avoid the recursion with a preallocated page pool, which is used to
split up large mappings and gets refilled in the return path of
kernel_map_pages after the split has been done. The size of the page
pool is adjusted to the available memory.

This part just implements the page pool and the initialization w/o
using it yet.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/asm-x86/cacheflush.h

@@ -44,6 +44,8 @@ int set_memory_np(unsigned long addr, int numpages);
 
 void clflush_cache_range(void *addr, unsigned int size);
 
+void cpa_init(void);
+
 #ifdef CONFIG_DEBUG_RODATA
 void mark_rodata_ro(void);
 #endif