ipmr: Convert ipmr_rtm_dumproute() to RCU.

ipmr: Convert ipmr_rtm_dumproute() to RCU.

ipmr_rtm_dumproute() calls mr_table_dump() or mr_rtm_dumproute(),
and mr_rtm_dumproute() finally calls mr_table_dump().

mr_table_dump() calls the passed function, _ipmr_fill_mroute().

_ipmr_fill_mroute() is a wrapper of ipmr_fill_mroute() to cast
struct mr_mfc * to struct mfc_cache *.

ipmr_fill_mroute() can be already called safely under RCU.

Let's convert ipmr_rtm_dumproute() to RCU.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260228221800.1082070-7-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>