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>

arch/x86/mm/init_32.c

@@ -664,6 +664,8 @@ void __init mem_init(void)
 	if (boot_cpu_data.wp_works_ok < 0)
 		test_wp_bit();
 
+	cpa_init();
+
 	/*
 	 * Subtle. SMP is doing it's boot stuff late (because it has to
 	 * fork idle threads) - but it also needs low mappings for the