Bluetooth: L2CAP: Try to allocate segment from the original pool

This makes the code attempt to allocate from the original buffer pool
before relying on le_data_pool which shall only be used as last resort
as it is configure with minimal possible buffers (1 per connection).

Change-Id: I85b581627f5c3b1bf1ee7c5fa69099c5aca13d4a
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>