Bluetooth: host: don't pull data if no view bufs

View buffers are now also a limited resource. Acquire them before
attempting to pull data. `CONFIG_BT_CONN_FRAG_COUNT` should be tuned on
a per-application basis to avoid this.

A possible optimization, that was present before, is to not create a
frag when the original buffer fits the controller's HCI size.

I prefer deferring this optimization to a future patchset.

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