The threshold for OOM protection is too small for systems with large
number of CPUs. Applications report ENOBUFs on connect() every 10
minutes.

The problem is that the variable net->xfrm.flow_cache_gc_count is a
global counter while the variable fc->high_watermark is a per-CPU
constant. Take the number of CPUs into account as well.

Fixes: 6ad3122a ("flowcache: Avoid OOM condition under preasure")
Reported-by: Lukáš Koldrt <lk@excello.cz>
Tested-by: Jan Hejl <jh@excello.cz>
Signed-off-by: Miroslav Urbanek <mu@miroslavurbanek.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>