flowcache: Avoid OOM condition under preasure

We can hit an OOM condition if we are under presure because
we can not free the entries in gc_list fast enough. So add
a counter for the not yet freed entries in the gc_list and
refuse new allocations if the value is too high.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
2 files changed