x86: defer cr3 reload when doing pud_clear()

PAE mode requires that we reload cr3 in order to guarantee that
changes to the pgd will be noticed by the processor.  This means that
in principle pud_clear needs to reload cr3 every time.  However,
because reloading cr3 implies a tlb flush, we want to avoid it where
possible.

pud_clear() is only used in a couple of places:
 - in free_pmd_range(), when pulling down a range of process address space, and
 - huge_pmd_unshare()

In both cases, the calling code will do a a tlb flush anyway, so
there's no need to do it within pud_clear().

In free_pmd_range(), the pud_clear is immediately followed by
pmd_free_tlb(); we can hook that to make the mmu_gather do an
unconditional full flush to make sure cr3 gets reloaded.

In huge_pmd_unshare, it is followed by flush_tlb_range, which always
results in a full cr3-reload tlb flush.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: Andi Kleen <ak@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: William Irwin <wli@holomorphy.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>