selftests/damon: fix unnecessary compilation warnings

selftests/damon: fix unnecessary compilation warnings

When testing overflow and overread, there is no need to keep unnecessary
compilation warnings, we should simply ignore them.

The motivation for this patch is to eliminate the compilation warning,
maybe one day we will compile the kernel with "-Werror -Wall", at which
point this compilation warning will turn into a compilation error, we
should fix this error in advance.

How to reproduce the problem (with gcc-11.3.1):

    $ make -C tools/testing/selftests/
    ...
    warning: `write' reading 4294967295 bytes from a region of size 1
    [-Wstringop-overread]
    warning: `read' writing 4294967295 bytes into a region of size 25
    overflows the destination [-Wstringop-overflow=]

"-Wno-stringop-overread" is supported at least in gcc-11.1.0.

Link: https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=d14c547abd484d3540b692bb8048c4a6efe92c8b
Link: https://lkml.kernel.org/r/tencent_51C4ACA8CB3895C2D7F35178440283602107@qq.com
Signed-off-by: Rong Tao <rongtao@cestc.cn>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>