stm: Potential read overflow in stm_char_policy_set_ioctl()

The "size" variable comes from the user so we need to verify that it's
large enough to hold an stp_policy_id struct.

Fixes: 7bd1d409 ("stm class: Introduce an abstraction for System Trace Module devices")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>