Patch from Dave Hansen <haveblue@us.ibm.com>

__pgd_offset() and pgd_offset() are completely different functions.
__pgd_offset() is really just a helper to figure out which entry in a
pgd an address would fall into.   pgd_offset() does all the leg work and
actually fetches the real pgd entry.

pgd_index() is a much saner name for what __pgd_offset() does.  In fact,
we do this:
#define __pgd_offset(address) pgd_index(address)

The attached patch removes all instances of __pgd_offset and just
replaces them with pgd_index.

Compiles with and without PAE on x86.