powerpc/cell/oprofile: Fix test on overlay_tbl_offset in vma_map

powerpc/cell/oprofile: Fix test on overlay_tbl_offset in vma_map

Offset is unsigned and when an address isn't found in the vma map
vma_map_lookup() returns the vma physical address + 0x10000000.

vma_map_lookup used to return 0xffffffff on a failed lookup, but
a change was made to return the vma physical address + 0x10000000
There are two callers of vam_map_lookup: one of them correctly
deals with this new return value, but the other (below) did not.

Signed-off-by: Roel Kluin <[email protected]>
Acked-by: Maynard Johnson <[email protected]>
Signed-off-by: Arnd Bergmann <[email protected]>
Signed-off-by: Benjamin Herrenschmidt <[email protected]>