ipv6: fix state corruption during proxy_ndp sysctl restart

ipv6: fix state corruption during proxy_ndp sysctl restart

When handling proxy_ndp, if rtnl_net_trylock() fails, the operation is
retried but as the value was already modified by the initial
proc_dointvec() call, the restarted syscall will read the newly modified
value as the 'old' state.

Fix this by taking the RTNL lock before parsing the input value if the
operation is a write.

Fixes: c92d5491a6d9 ("netconf: add support for IPv6 proxy_ndp")
Reviewed-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260622130857.5115-6-fmancera@suse.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>