Bluetooth: ATT: lock scheduler when sending from user thread

`att_req_send_process` is not thread safe.

In the case where the current context is pre-emptible (e.g. when a user
sends something over GATT from the main thread):

The iterator in `att_req_send_process` can get interrupted mid-processing,
breaking the logic and resulting in an assert/crash/data corruption.

Additionally, the connection state check in this fn is also not
thread-safe, the RX thread could pre-empt us and disconnect right before
we attempt to send on an ATT bearer that is on it.

This is a hotfix until we have a generalized solution for the host API
surface. It seems a lot of our logic assumes cooperative priority.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>