kemleak-test: build as module only

mm/kmemleak-test.c is used to provide an example of how kmemleak
tool works.

Memory is leaked at module unload-time, so building the test
in kernel (Y) makes the leaks impossible and the test useless.

Qualify DEBUG_KMEMLEAK_TEST config symbol with "depends on m",
to restrict module-only building.

Signed-off-by: Daniel Baluta <dbaluta@ixiacom.com>
Acked-by: Pekka Enberg <penberg@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>