When using 4level-fixup.h, a PMD page may end up beeing freed before the
matching PGD entry is cleared due to the way the compatibility macros
work. This can cause nasty races on some architectures.

This patch fixes it by defining pud_clear() to be pgd_clear(). That
means we'll actually write 0 twice, a small price to pay here,
especially seeing how easy it is to convert to the new headers anyway
(hint hint, ppc & ppc64 patches as soon as 2.6.11 is out).
Signed-off-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>