preload: truncate for paths outside of pmemfile pool is handled incorrectly

[Marcin Ślusarz]

Code from hook_truncate: "return syscall_no_intercept(SYS_truncate, where.at.kernel_fd, where.path, length);" is incorrect. Truncate syscall accepts only 2 arguments (and there's no truncateat, so it's not a simple fix).