ipv6: mcast: add RCU protection to mld_newpack()

A race condition in Linux kernel's IPv6 multicast code where mld_newpack() function could be called without proper synchronization protection, potentially causing memory corruption or crashes when multiple threads access it simultaneously.

The mld_newpack() function in IPv6 multicast handling lacked RCU (Read-Copy-Update) protection when allocating socket buffers, creating a race condition when concurrent calls occurred without RTNL lock. The fix implements RCU protection for socket charging and replaces sock_alloc_send_skb() with alloc_skb() to avoid blocking operations, preventing data structure inconsistency and potential kernel panic.