mm/Makefile: disable KCSAN for kmemleak

Kmemleak could scan task stacks while plain writes happens to those stack
variables which could results in data races.  For example, in
sys_rt_sigaction and do_sigaction(), it could have plain writes in a
32-byte size.  Since the kmemleak does not care about the actual values of
a non-pointer and all do_sigaction() call sites only copy to stack
variables, just disable KCSAN for kmemleak to avoid annotating anything
outside Kmemleak just because Kmemleak scans everything.

Suggested-by: Marco Elver <elver@google.com>
Signed-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Marco Elver <elver@google.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: http://lkml.kernel.org/r/1583263716-25150-1-git-send-email-cai@lca.pw


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>