netfilter: core: Fix clang warnings about unused static inlines

Unlike gcc, clang warns about unused static inlines that are not in an
include file:

  net/netfilter/core.c:344:20: error: unused function 'nf_ingress_hook' [-Werror,-Wunused-function]
  static inline bool nf_ingress_hook(const struct nf_hook_ops *reg, int pf)
                     ^
  net/netfilter/core.c:353:20: error: unused function 'nf_egress_hook' [-Werror,-Wunused-function]
  static inline bool nf_egress_hook(const struct nf_hook_ops *reg, int pf)
                     ^

According to commit 6863f564 ("kbuild: allow Clang to find unused
static inline functions for W=1 build"), the proper resolution is to
mark the affected functions as __maybe_unused.  An alternative approach
would be to move them to include/linux/netfilter_netdev.h, but since
Pablo didn't do that in commit ddcfa710 ("netfilter: add
nf_ingress_hook() helper function"), I'm guessing __maybe_unused is
preferred.

This fixes both the warning introduced by Pablo in v5.10 as well as the
one recently introduced by myself with commit 42df6e1d ("netfilter:
Introduce egress hook").

Fixes: ddcfa710

 ("netfilter: add nf_ingress_hook() helper function")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>