kselftest/arm64: mte: user_mem: Fix write() warning

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 "val".

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Mark Brown <broone@kernel.org>
Link: https://lore.kernel.org/r/20210319165334.29213-5-andre.przywara@arm.com


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