llc: hold llc_sap before release_sock()

syzbot reported we still access llc->sap in llc_backlog_rcv()
after it is freed in llc_sap_remove_socket():

Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1b9/0x294 lib/dump_stack.c:113
 print_address_description+0x6c/0x20b mm/kasan/report.c:256
 kasan_report_error mm/kasan/report.c:354 [inline]
 kasan_report.cold.7+0x242/0x2fe mm/kasan/report.c:412
 __asan_report_load1_noabort+0x14/0x20 mm/kasan/report.c:430
 llc_conn_ac_send_sabme_cmd_p_set_x+0x3a8/0x460 net/llc/llc_c_ac.c:785
 llc_exec_conn_trans_actions net/llc/llc_conn.c:475 [inline]
 llc_conn_service net/llc/llc_conn.c:400 [inline]
 llc_conn_state_process+0x4e1/0x13a0 net/llc/llc_conn.c:75
 llc_backlog_rcv+0x195/0x1e0 net/llc/llc_conn.c:891
 sk_backlog_rcv include/net/sock.h:909 [inline]
 __release_sock+0x12f/0x3a0 net/core/sock.c:2335
 release_sock+0xa4/0x2b0 net/core/sock.c:2850
 llc_ui_release+0xc8/0x220 net/llc/af_llc.c:204

llc->sap is refcount'ed and llc_sap_remove_socket() is paired
with llc_sap_add_socket(). This can be amended by holding its refcount
before llc_sap_remove_socket() and releasing it after release_sock().

Reported-by:  <syzbot+6e181fc95081c2cf9051@syzkaller.appspotmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>