phonet: pep: fix use-after-free in pep_get_sb()

In the Linux kernel, the following vulnerability has been resolved: phonet: pep: fix use-after-free in pep_get_sb() pep_get_sb() doesn't consider that pskb_may_pull() might have relocated the skb data, and continue to access the older pointer, causing UAF. Reproduced under KASAN: BUG: KASAN: slab-use-after-free in pep_get_sb+0x234/0x3b0 Read of size 1 at addr ff11000105510f50 by task repro/157 pep_get_sb+0x234/0x3b0 pipe_handler_do_rcv+0x5f7/0xa10 pep_do_rcv+0x203/0x410 __sk_receive_skb+0x471/0x4a0 phonet_rcv+0x5b3/0x6c0 __netif_receive_skb+0xcc/0x1d0 Refetch the header with skb_header_pointer() after pskb_may_pull(), so the possibly stale pointer is no longer dereferenced. There are better ways to solve this, but, this is the less instrusive one.