kselftest/arm64: mte: ksm_options: Fix fscanf warning

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

Make GCC happy by checking whether we actually parsed one integer.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Mark Brown <broone@kernel.org>
Link: https://lore.kernel.org/r/20210319165334.29213-4-andre.przywara@arm.com


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