irda: precedence bug in irlmp_seq_hb_idx()

[ Upstream commit 50010c20597d14667eff0fdb628309986f195230 ]

This is decrementing the pointer, instead of the value stored in the
pointer.  KASan detects it as an out of bounds reference.

Reported-by: "Berry Cheng 程君(成淼)" <chengmiao.cj@alibaba-inc.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --cc net/irda/irlmp.c
Simple merge