ipmr: Add __rcu to netns_ipv4.mrt.

ipmr: Add __rcu to netns_ipv4.mrt.

kernel test robot reported this Sparse warning:

  $ make C=1 net/ipv4/ipmr.o
  net/ipv4/ipmr.c:312:24: error: incompatible types in comparison expression (different address spaces):
  net/ipv4/ipmr.c:312:24:    struct mr_table [noderef] __rcu *
  net/ipv4/ipmr.c:312:24:    struct mr_table *

Let's add __rcu annotation to netns_ipv4.mrt.

Fixes: b3b6babf4751 ("ipmr: Free mr_table after RCU grace period.")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202605030032.glNApko7-lkp@intel.com/
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260502180755.359554-1-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>