[x86] Make split_page_count check for empty levels.

[Dave Jones <davej@codemonkey.org.uk>]

In PAGEALLOC_DEBUG builds, we disable the 2M pages.
Unfortunatly when we split up mappings during boot,
split_page_count doesn't take this into account, and
starts decrementing an empty direct_pages_count level.

This results in /proc/meminfo showing crazy things like
DirectMap2M:    18446744073709543424 kB

Signed-off-by: Dave Jones <davej@codemonkey.org.uk>

diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index a3137a4feed1..ff0e0c6c350e 100644
--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -66,6 +66,9 @@ void update_page_count(int level, unsigned long pages)
 
 static void split_page_count(int level)
 {
+	if (direct_pages_count[level] == 0)
+		return;
+
 	direct_pages_count[level]--;
 	direct_pages_count[level - 1] += PTRS_PER_PTE;
 }