Bluetooth: ATT: Fix using k_fifo API with net_buf

net_buf shall not be used with k_fifo since net_buf_unref will assume
unused bytes in the beginning are actually fragments causing it to
unref them as well.

Jira: ZEP-1489

Change-Id: I5ce420de73b245dc20eb15ea4d8d0b6ba346e513
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>