nexthop: Only parse NHA_OP_FLAGS for dump messages that require it

The attribute is parsed in __nh_valid_dump_req() which is called by the
dump handlers of RTM_GETNEXTHOP and RTM_GETNEXTHOPBUCKET although it is
only used by the former and rejected by the policy of the latter.

Move the parsing to nh_valid_dump_req() which is only called by the dump
handler of RTM_GETNEXTHOP.

This is a preparation for a subsequent patch.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20240311162307.545385-3-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>