mm: use vm_unmapped_area() on powerpc architecture

mm: use vm_unmapped_area() on powerpc architecture

Update the powerpc slice_get_unmapped_area function to make use of
vm_unmapped_area() instead of implementing a brute force search.

Signed-off-by: Michel Lespinasse <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Paul Mackerras <[email protected]>
Tested-by: "Aneesh Kumar K.V" <[email protected]>
Acked-by: David Gibson <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Benjamin Herrenschmidt <[email protected]>