powerpc: Fix bad pmd error with book3E config

Book3E uses the hugepd at PMD level and don't encode pte directly
at the pmd level. So it will find the lower bits of pmd set
and the pmd_bad check throws error. Infact the current code
will never take the free_hugepd_range call at all because it will
clear the pmd if it find a hugepd pointer. Fix this by clearing
bad pmd only if it is not a hugepd pointer.

This is regression introduced by e2b3d202
"powerpc: Switch 16GB and 16MB explicit hugepages to a different page table format"

Reported-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>