Bluetooth: L2CAP: fix UAF in l2cap_le_connect_rsp

Bluetooth: L2CAP: fix UAF in l2cap_le_connect_rsp

l2cap_le_connect_rsp() obtains a channel via
__l2cap_get_chan_by_ident() but neither holds a reference nor uses
l2cap_chan_hold_unless_zero() before locking and operating on it.
A concurrent l2cap_chan_del() triggered by a remote disconnect can
free the channel between the lookup and l2cap_chan_lock(), causing
a use-after-free.

The BR/EDR counterpart l2cap_connect_rsp() and the sibling handler
l2cap_le_command_rej() already use l2cap_chan_hold_unless_zero()
to safely hold a reference, but l2cap_le_connect_rsp() was left
unprotected.

Fix by adding l2cap_chan_hold_unless_zero() after the ident lookup
and l2cap_chan_put() on the exit path, consistent with other L2CAP
response handlers.

Fixes: f1496dee9cbd ("Bluetooth: Add initial code for LE L2CAP Connect Request")
Assisted-by: Claude:deepseek-v4-pro
Signed-off-by: Jiale Yao <yaojiale02@163.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>