net: buf: Fix warnings related to legacy API

This removes the use of sys_thread_self_get replacing with
k_current_get and also remove the use __func__ macro as that is already
added by SYS_LOG macros.

Change-Id: I9949896c76070183b3c1ea40100d82291748fc68
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>