Merge branch 'nexthop-convert-rtm_-new-del-nexthop-to-per-netns-rtnl'

Kuniyuki Iwashima says:

====================
nexthop: Convert RTM_{NEW,DEL}NEXTHOP to per-netns RTNL.

Patch 1 - 5 move some validation for RTM_NEWNEXTHOP so that it can be
called without RTNL.

Patch 6 & 7 converts RTM_NEWNEXTHOP and RTM_DELNEXTHOP to per-netns RTNL.

Note that RTM_GETNEXTHOP and RTM_GETNEXTHOPBUCKET are not touched in
this series.

rtm_get_nexthop() can be easily converted to RCU, but rtm_dump_nexthop()
needs more work due to the left-to-right rbtree walk, which looks prone
to node deletion and tree rotation without a retry mechanism.

v1: https://lore.kernel.org/netdev/20250318233240.53946-1-kuniyu@amazon.com/
====================

Link: https://patch.msgid.link/20250319230743.65267-1-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>