netfilter: nf_tables: Only use nf_skip_indirect_calls() when MITIGATION_RETPOLINE

1. MITIGATION_RETPOLINE is x86-only (defined in arch/x86/Kconfig),
so no need to AND with CONFIG_X86 when checking if enabled.

2. Remove unused declaration of nf_skip_indirect_calls() when
MITIGATION_RETPOLINE is disabled to avoid warnings.

3. Declare nf_skip_indirect_calls() and nf_skip_indirect_calls_enable()
as inline when MITIGATION_RETPOLINE is enabled, as they are called
only once and have simple logic.

Fix follow error with clang-21 when W=1e:
  net/netfilter/nf_tables_core.c:39:20: error: unused function 'nf_skip_indirect_calls' [-Werror,-Wunused-function]
     39 | static inline bool nf_skip_indirect_calls(void) { return false; }
        |                    ^~~~~~~~~~~~~~~~~~~~~~
  1 error generated.
  make[4]: *** [scripts/Makefile.build:207: net/netfilter/nf_tables_core.o] Error 1
  make[3]: *** [scripts/Makefile.build:465: net/netfilter] Error 2
  make[3]: *** Waiting for unfinished jobs....

Fixes: d8d76062 ("netfilter: nf_tables: add static key to skip retpoline workarounds")
Co-developed-by: Wentao Guan <guanwentao@uniontech.com>
Signed-off-by: Wentao Guan <guanwentao@uniontech.com>
Signed-off-by: WangYuli <wangyuli@uniontech.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>