Bluetooth: Use GFP_KERNEL in mgmt_pending_add

We are allowed to sleep in mgmt_pending_add, so we should use
GFP_KERNEL for memory allocations instead of GFP_ATOMIC.

Signed-off-by: Andre Guedes <aguedespe@gmail.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>