netfilter: annotate NAT helper hook pointers with __rcu

netfilter: annotate NAT helper hook pointers with __rcu

The NAT helper hook pointers are updated and dereferenced under RCU rules,
but lack the proper __rcu annotation.

This makes sparse report address space mismatches when the hooks are used
with rcu_dereference().

Add the missing __rcu annotations to the global hook pointer declarations
and definitions in Amanda, FTP, IRC, SNMP and TFTP.

No functional change intended.

Suggested-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>