bpf, doc: Remove references to warning message when using bpf_trace_printk()

mainline inclusion
from mainline-v5.9-rc1
commit 7fb20f9e
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I4PKDW


CVE: NA

--------------------------------

The BPF helper bpf_trace_printk() no longer uses trace_printk();
it is now triggers a dedicated trace event.  Hence the described
warning is no longer present, so remove the discussion of it as
it may confuse people.

Fixes: ac5a72ea ("bpf: Use dedicated bpf_trace_printk event instead of trace_printk()")
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/1596801029-32395-1-git-send-email-alan.maguire@oracle.com


Signed-off-by: Liu Xinpeng <liuxp11@chinatelecom.cn>	# openEuler_contributor
Signed-off-by: Ctyun Kernel <ctyuncommiter01@chinatelecom.cn>	# openEuler_contributor
Signed-off-by: Pu Lehui <pulehui@huawei.com>
Reviewed-by: Yang Jihong <yangjihong1@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>