linux/netfilter.h: fix kernel-doc warnings

kernel-doc does not support DECLARE_PER_CPU(), so don't mark it with
kernel-doc notation.

One comment block is not kernel-doc notation, so just use
"/*" to begin the comment.

Quietens these warnings:

netfilter.h:493: warning: Function parameter or member 'bool' not described in 'DECLARE_PER_CPU'
netfilter.h:493: warning: Function parameter or member 'nf_skb_duplicated' not described in 'DECLARE_PER_CPU'
netfilter.h:493: warning: expecting prototype for nf_skb_duplicated(). Prototype was for DECLARE_PER_CPU() instead
netfilter.h:496: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Contains bitmask of ctnetlink event subscribers, if any.

Fixes: e7c8899f ("netfilter: move tee_active to core")
Fixes: fdf64911 ("netfilter: ctnetlink: make event listener tracking global")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>