[POWERPC] Fix bogus BUG_ON() in in hugetlb_get_unmapped_area()

The powerpc specific version of hugetlb_get_unmapped_area() makes some
unwarranted assumptions about what checks have been made to its
parameters by its callers.  This will lead to a BUG_ON() if a 32-bit
process attempts to make a hugepage mapping which extends above
TASK_SIZE (4GB).

I'm not sure if these assumptions came about because they were valid
with earlier versions of the get_unmapped_area() path, or if it was
always broken.  Nonetheless this patch fixes the logic, and removes
the crash.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>