Bluetooth: GATT: Fix gatt buffer leak for write commands and notify

Fix GATT buffer leak when bt_att_send returns error the allocated
buffer is never freed. Discovered case where the link was disconnected
during the function call, so when GATT checkd the link was still
connected, but ATT checkd the link was disconnected.

Fixes: #19889

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>