Bluetooth: Exclude released devices from RFCOMMGETDEVLIST ioctl

When enumerating RFCOMM devices in the rfcomm_dev_list, holding
the rfcomm_dev_lock only guarantees the existence of the enumerated
rfcomm_dev in memory, and not safe access to its state. Testing
the device state (such as RFCOMM_TTY_RELEASED) does not guarantee
the device will remain in that state for the subsequent access
to the rfcomm_dev's fields, nor guarantee that teardown has not
commenced.

Obtain an rfcomm_dev reference for the duration of rfcomm_dev
access.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Tested-By: Alexander Holler <holler@ahsoftware.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>