Bluetooth: 6lowpan: Remove unnecessary RCU callback

When kfree() is all that's needed to free an object protected by RCU
there's a kfree_rcu() convenience function that can be used. This patch
updates the 6lowpan code to use this, thereby eliminating the need for
the separate peer_free() function.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>