soc: qcom: socinfo: Avoid out of bounds read of serial number

In the Linux kernel, the following vulnerability has been resolved:

soc: qcom: socinfo: Avoid out of bounds read of serial number

On MSM8916 devices, the serial number exposed in sysfs is constant and does

not change across individual devices. It's always:

db410c:/sys/devices/soc0$ cat serial_number

2644893864

The firmware used on MSM8916 exposes SOCINFO_VERSION(0, 8), which does not

have support for the serial_num field in the socinfo struct. There is an

existing check to avoid exposing the serial number in that case, but it's

not correct: When checking the item_size returned by SMEM, we need to make

sure the *end* of the serial_num is within bounds, instead of comparing

with the *start* offset. The serial_number currently exposed on MSM8916

devices is just an out of bounds read of whatever comes after the socinfo

struct in SMEM.

Fix this by changing offsetof() to offsetofend(), so that the size of the

field is also taken into account.