net: seg6: initialize induction variable to first valid array index

Fixes the following warnings observed when building
CONFIG_IPV6_SEG6_LWTUNNEL=y with clang:

  net/ipv6/seg6_local.o: warning: objtool: seg6_local_fill_encap() falls
  through to next function seg6_local_get_encap_size()
  net/ipv6/seg6_local.o: warning: objtool: seg6_local_cmp_encap() falls
  through to next function input_action_end()

LLVM can fully unroll loops in seg6_local_get_encap_size() and
seg6_local_cmp_encap(). One issue in those loops is that the induction
variable is initialized to 0. The loop iterates over members of
seg6_action_params, a global array of struct seg6_action_param calling
their put() function pointer members.  seg6_action_param uses an array
initializer to initialize SEG6_LOCAL_SRH and later elements, which is
the third enumeration of an anonymous union.

The guard `if (attrs & SEG6_F_ATTR(i))` may prevent this from being
called at runtime, but it would still be UB for
`seg6_action_params[0]->put` to be called; the unrolled loop will make
the initial iterations unreachable, which LLVM will later rotate to
fallthrough to the next function.

Make this more obvious that this cannot happen to the compiler by
initializing the loop induction variable to the minimum valid index that
seg6_action_params is initialized to.

Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/r/20220802161203.622293-1-ndesaulniers@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>