net: add sanity check in proto_register()

prot->memory_allocated should only be set if prot->sysctl_mem
is also set.

This is a followup of commit 25206111

 ("crypto: af_alg - get
rid of alg_memory_allocated").

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20220216171801.3604366-1-eric.dumazet@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>