ipv6: mcast: use min() to simplify the code

When coping sockaddr in ip6_mc_msfget(), the time of copies
depends on the minimum value between sl_count and gf_numsrc.
Using min() here is very semantic.

Signed-off-by: Li Zetao <lizetao1@huawei.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240822133908.1042240-7-lizetao1@huawei.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>