ipv6: fix array index in ip6_mc_add_src()

Convert array index from the loop bound to the loop index.

And remove the void type conversion to ip6_mc_del1_src() return
code, seem it is unnecessary, since ip6_mc_del1_src() does not
use __must_check similar attribute, no compiler will report the
warning when it is removed.

v2: enrich the commit header

Signed-off-by: RongQing.Li <roy.qing.li@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>