Bluetooth: Serialize RFCOMMCREATEDEV and RFCOMMRELEASEDEV ioctls

At least two different race conditions exist with multiple concurrent
RFCOMMCREATEDEV and RFCOMMRELEASEDEV ioctls:
* Multiple concurrent RFCOMMCREATEDEVs with RFCOMM_REUSE_DLC can
  mistakenly share the same DLC.
* RFCOMMRELEASEDEV can destruct the rfcomm_dev still being
  constructed by RFCOMMCREATEDEV.

Introduce rfcomm_ioctl_mutex to serialize these add/remove operations.

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