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

tools/testing/selftests/arm64/mte/check_user_mem.c

@@ -33,7 +33,8 @@ static int check_usermem_access_fault(int mem_type, int mode, int mapping)
 	if (fd == -1)
 		return KSFT_FAIL;
 	for (i = 0; i < len; i++)
-		write(fd, &val, sizeof(val));
+		if (write(fd, &val, sizeof(val)) != sizeof(val))
+			return KSFT_FAIL;
 	lseek(fd, 0, 0);
 	ptr = mte_allocate_memory(len, mem_type, mapping, true);
 	if (check_allocated_memory(ptr, len, mem_type, true) != KSFT_PASS) {