kselftest/arm64: mte: Fix clang warning

kselftest/arm64: mte: Fix clang warning

if (!prctl(...) == 0) is not only cumbersome to read, it also upsets
clang and triggers a warning:
------------
mte_common_util.c:287:6: warning: logical not is only applied to the
left hand side of this comparison [-Wlogical-not-parentheses]
....

Fix that by just comparing against "not 0" instead.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Mark Brown <broone@kernel.org>
Link: https://lore.kernel.org/r/20210319165334.29213-11-andre.przywara@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>