x86: return conditional to mmu

Just return our allocation if we don't have an mmu. For i386, where this patch
is being applied, we never have. So our goal is just to have the code to look like
x86_64's.

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>