ipv6: mcast: Remove mca_get().

ipv6: mcast: Remove mca_get().

Since commit 63ed8de4be81 ("mld: add mc_lock for protecting per-interface
mld data"), the newly allocated struct ifmcaddr6 cannot be removed until
inet6_dev->mc_lock is released, so mca_get() and mc_put() are unnecessary.

Let's remove the extra refcounting.

Note that mca_get() was only used in __ipv6_dev_mc_inc().

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250702230210.3115355-5-kuni1840@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>