powerpc/mm/hash: Handle mmap_min_addr correctly in get_unmapped_area topdown search

When doing top-down search the low_limit is not PAGE_SIZE but rather
max(PAGE_SIZE, mmap_min_addr). This handle cases in which mmap_min_addr >
PAGE_SIZE.

Fixes: fba2369e ("mm: use vm_unmapped_area() on powerpc architecture")
Reviewed-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>