Bluetooth: Remove GFP_ATOMIC usage from l2cap_core.c

Since we change the Bluetooth core to run in process context we don't need
to use GFP_ATOMIC in many of places we were using it. The we just replace
by GFP_KERNEL.

Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>