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

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

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

  * mroute_do_pim
  * mroute_do_assert
  * mroute_do_wrvifwhole

However, ip_mroute_getsockopt() does not hold RTNL and read the first
two fields locklessly, and ip_mr_forward() reads all the three under
RCU.  pim_rcv_v1() also reads mroute_do_pim locklessly.

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

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260228221800.1082070-3-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>