Bluetooth: host: copy fragment data at the last minute

Only copy the data from the 'parent' buf into the segment buf if we
successfully acquired the HCI semaphore (ie there are available
controller buffers).

This way we avoid pulling and pushing back the data in the case where
there aren't any controller buffers available.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>
(cherry picked from commit ca51439c)