net: add a route cache full diagnostic message

stable inclusion
from stable-v4.19.304
commit 47468fae2704151503214f4d4327c164118247fd
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I90HST

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=47468fae2704151503214f4d4327c164118247fd



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

commit 22c2ad61 upstream.

In some testing scenarios, dst/route cache can fill up so quickly
that even an explicit GC call occasionally fails to clean it up. This leads
to sporadically failing calls to dst_alloc and "network unreachable" errors
to the user, which is confusing.

This patch adds a diagnostic message to make the cause of the failure
easier to determine.

Signed-off-by: Peter Oskolkov <posk@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Suraj Jitindar Singh <surajjs@amazon.com>
Cc: <stable@vger.kernel.org> # 4.19.x
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Dong Chenchen <dongchenchen2@huawei.com>