flowcache: restore a single flow_cache kmem_cache

It is not legal to create multiple kmem_cache having the same name.

flowcache can use a single kmem_cache, no need for a per netns
one.

Fixes: ca925cf1 ("flowcache: Make flow cache name space aware")
Reported-by: Jakub Kicinski <moorray3@wp.pl>
Tested-by: Jakub Kicinski <moorray3@wp.pl>
Tested-by: Fan Du <fan.du@windriver.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>