linux-user/syscall: Fix missing target_to_host_timespec64() check

Coverity pointed out (CID 1432339) that target_to_host_timespec64() can
fail with -TARGET_EFAULT but we never check the return value. This patch
checks the return value and handles the error.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <cad74fae734d2562746b94acd9c34b00081c89bf.1604432881.git.alistair.francis@wdc.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>