ipmr: convert /proc handlers to rcu_read_lock()

[ Upstream commit b96ef16d ]

We can use standard rcu_read_lock(), to get rid
of last read_lock(&mrt_lock) call points.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: fc9c273d ("ipmr: fix tables suspicious RCU usage")
Signed-off-by: Sasha Levin <sashal@kernel.org>