nf: IDLETIMER: Use fullsock when querying uid

nf: IDLETIMER: Use fullsock when querying uid

sock_i_uid() acquires the sk_callback_lock which does not exist for
sockets in TCP_NEW_SYN_RECV state. This results in errors showing up
as spinlock bad magic. Fix this by looking for the full sock as
suggested by Eric.

Callstack for reference -

-003|rwlock_bug
-004|arch_read_lock
-004|do_raw_read_lock
-005|raw_read_lock_bh
-006|sock_i_uid
-007|from_kuid_munged(inline)
-007|reset_timer
-008|idletimer_tg_target
-009|ipt_do_table
-010|iptable_mangle_hook
-011|nf_iterate
-012|nf_hook_slow
-013|NF_HOOK_COND(inline)
-013|ip_output
-014|ip_local_out
-015|ip_build_and_send_pkt
-016|tcp_v4_send_synack
-017|atomic_sub_return(inline)
-017|reqsk_put(inline)
-017|tcp_conn_request
-018|tcp_v4_conn_request
-019|tcp_rcv_state_process
-020|tcp_v4_do_rcv
-021|tcp_v4_rcv
-022|ip_local_deliver_finish
-023|NF_HOOK_THRESH(inline)
-023|NF_HOOK(inline)
-023|ip_local_deliver
-024|ip_rcv_finish
-025|NF_HOOK_THRESH(inline)
-025|NF_HOOK(inline)
-025|ip_rcv
-026|deliver_skb(inline)
-026|deliver_ptype_list_skb(inline)
-026|__netif_receive_skb_core
-027|__netif_receive_skb
-028|netif_receive_skb_internal
-029|netif_receive_skb

Change-Id: Ic8f3a3d2d7af31434d1163b03971994e2125d552
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
Cc: Eric Dumazet <edumazet@google.com>