x86/mm: Fix 'no5lvl' handling

early_identify_cpu() has to use early version of pgtable_l5_enabled()
that doesn't rely on cpu_feature_enabled().

Defining USE_EARLY_PGTABLE_L5 before all includes does the trick.

I lost the define in one of reworks of the original patch.

Fixes: 372fddf7 ("x86/mm: Introduce the 'no5lvl' kernel parameter")
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lkml.kernel.org/r/20180622220841.54135-3-kirill.shutemov@linux.intel.com