x86: fix early_ioremap()/btmap

fix a long-standing weakness of the early-ioremap allocator: it
uses a single pgd entry for the boot mappings, and was not properly
protecting itself against crossing a 2MB (4MB) boundary.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>