mtd: mtd_oobtest: Fix the address offset with vary_offset case

When vary_offset is set (e.g. test case 3), the offset is not always
zero so memcmpshow() will show the wrong offset in the print message.
To fix this we introduce a new function memcmpshowoffset() which takes
offset as a parameter and displays the right offset and use it in
the case where offset is non zero.

The old memcmpshow() functionality is preserved by converting it into
a macro with offset preset to 0.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>