net/smc: check smcd_v2_ext_offset when receiving proposal msg

In the Linux kernel, the following vulnerability has been resolved: net/smc: check smcd_v2_ext_offset when receiving proposal msg When receiving proposal msg in server, the field smcd_v2_ext_offset in proposal msg is from the remote client and can not be fully trusted. Once the value of smcd_v2_ext_offset exceed the max value, there has the chance to access wrong address, and crash may happen. This patch checks the value of smcd_v2_ext_offset before using it.