ip6mr: Annotate access to mrt->mroute_do_{pim,assert,wrvifwhole}.

ip6mr: Annotate access to mrt->mroute_do_{pim,assert,wrvifwhole}.

These fields in struct mr_table are updated in ip6_mroute_setsockopt()
under RTNL:

  * mroute_do_pim
  * mroute_do_assert (MRT6_PIM is under RTNL while MRT6_ASSERT is lockless)
  * mroute_do_wrvifwhole

However, ip6_mroute_getsockopt() does not hold RTNL and read the first
two fields locklessly, and ip6_mr_forward() reads all the three under
RCU.

Let's use WRITE_ONCE() and READ_ONCE() for them.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260604224712.3209821-3-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>