netfilter: use function typedefs for __rcu NAT helper hook pointers

netfilter: use function typedefs for __rcu NAT helper hook pointers

After commit 07919126ecfc ("netfilter: annotate NAT helper hook pointers
with __rcu"), sparse can warn about type/address-space mismatches when
RCU-dereferencing NAT helper hook function pointers.

The hooks are __rcu-annotated and accessed via rcu_dereference(), but the
combination of complex function pointer declarators and the WRITE_ONCE()
machinery used by RCU_INIT_POINTER()/rcu_assign_pointer() can confuse
sparse and trigger false positives.

Introduce typedefs for the NAT helper function types, so __rcu applies to
a simple "fn_t __rcu *" pointer form. Also replace local typeof(hook)
variables with "fn_t *" to avoid propagating __rcu address space into
temporaries.

No functional change intended.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202603022359.3dGE9fwI-lkp@intel.com/
Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>