Bluetooth: Host: K_NO_WAIT in bt_att_req_alloc() in SYS WQ

This commit prevents ATT request APIs from blocking waiting on the
req_slab pool on the system work queue. The API will instead return
-ENOMEM.

This aligns with commit 05b16b97, which
establishes that the GATT request APIs are non-blocking on the system
work queue. That commit makes GATT request APIs fail with -ENOMEM when
they fail to allocate a buffer for the ATT PDU on the system work queue.

There is no reason to make this distinction between the two resources,
and this makes the API more consistent.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>