vm_read_mem: detect address truncation

vm_read_mem: detect address truncation

When sizeof(kernel_ureg_t) > sizeof(long), the tracee address passed
to vm_read_mem could be silently truncated.  Detect this situation
and return EIO when the tracee address does not fit into unsigned long.

* util.c (vm_read_mem): Save raddr argument into a temporary variable
truncated_raddr of type unsigned long.  Set errno to EIO and return -1
when truncated_raddr does not equal to raddr.