net/smc: check iparea_offset and ipv6_prefixes_cnt when receiving proposal msg

In the Linux kernel, the following vulnerability has been resolved: net/smc: check iparea_offset and ipv6_prefixes_cnt when receiving proposal msg When receiving proposal msg in server, the field iparea_offset and the field ipv6_prefixes_cnt in proposal msg are from the remote client and can not be fully trusted. Especially the field iparea_offset, once exceed the max value, there has the chance to access wrong address, and crash may happen. This patch checks iparea_offset and ipv6_prefixes_cnt before using them.