x86/mm: Stop pretending pgtable_l5_enabled is a variable

pgtable_l5_enabled is defined using cpu_feature_enabled() but we refer
to it as a variable. This is misleading.

Make pgtable_l5_enabled() a function.

We cannot literally define it as a function due to circular dependencies
between header files. Function-alike macros is close enough.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180518103528.59260-4-kirill.shutemov@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/arch/x86/mm/ident_map.c b/arch/x86/mm/ident_map.c
index a2f0c7e..fe7a125 100644
--- a/arch/x86/mm/ident_map.c
+++ b/arch/x86/mm/ident_map.c
@@ -123,7 +123,7 @@ int kernel_ident_mapping_init(struct x86_mapping_info *info, pgd_t *pgd_page,
 		result = ident_p4d_init(info, p4d, addr, next);
 		if (result)
 			return result;
-		if (pgtable_l5_enabled) {
+		if (pgtable_l5_enabled()) {
 			set_pgd(pgd, __pgd(__pa(p4d) | info->kernpg_flag));
 		} else {
 			/*