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 <12o3l@tiscali.nl>
Acked-by: Maynard Johnson <maynardj@us.ibm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>