kcm: lock lower socket in kcm_attach

Need to lock lower socket in order to provide mutual exclusion
with kcm_unattach.

v2: Add Reported-by for syzbot

Fixes: ab7ac4eb

 ("kcm: Kernel Connection Multiplexor module")
Reported-by:  <syzbot+ea75c0ffcd353d32515f064aaebefc5279e6161e@syzkaller.appspotmail.com>
Signed-off-by: Tom Herbert <tom@quantonium.net>
Signed-off-by: David S. Miller <davem@davemloft.net>