kselftest/arm64: mte: common: Fix write() warnings

Out of the box Ubuntu's 20.04 compiler warns about missing return value
checks for write() (sys)calls.

Make GCC happy by checking whether we actually managed to write out our
buffer.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Mark Brown <broone@kernel.org>
Link: https://lore.kernel.org/r/20210319165334.29213-6-andre.przywara@arm.com


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>