At present, the powerpc pmd_bad() and pud_bad() macros return false
unless the given pmd or pud is zero.  This patch makes these tests
more thorough, checking if the given pmd or pud looks like a plausible
pte page or pmd page pointer respectively.  This can result in helpful
error messages when messing with the pagetable code.
Signed-off-by: David Gibson <dwg@au1.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>