lib/test_string.c: avoid masking memset16/32/64 failures

lib/test_string.c: avoid masking memset16/32/64 failures

[ Upstream commit 33d6e0ff68af74be0c846c8e042e84a9a1a0561e ]

If a memsetXX implementation is completely broken and fails in the first
iteration, when i, j, and k are all zero, the failure is masked as zero
is returned.  Failing in the first iteration is perhaps the most likely
failure, so this makes the tests pretty much useless.  Avoid the
situation by always setting a random unused bit in the result on
failure.

Link: http://lkml.kernel.org/r/20190506124634.6807-3-peda@axentia.se
Fixes: 03270c13c5ff ("lib/string.c: add testcases for memset16/32/64")
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>