ipv6: eliminate ndisc_ops_is_useropt()

as it doesn't seem to offer anything of value.

There's only 1 trivial user:
int lowpan_ndisc_is_useropt(u8 nd_opt_type) {
return nd_opt_type == ND_OPT_6CO;
}

but there's no harm to always treating that as
a useropt...

Cc: David Ahern <[e-mail]>
Cc: YOSHIFUJI Hideaki / 吉藤英明 <[e-mail]>
Signed-off-by: Maciej Żenczykowski <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/net/ndisc.h

···
  * The following hooks can be defined; unless noted otherwise, they are
  * optional and can be filled with a null pointer.
  *
- * int (*is_useropt)(u8 nd_opt_type):
- *     This function is called when IPv6 decide RA userspace options. if
- *     this function returns 1 then the option given by nd_opt_type will
- *     be handled as userspace option additional to the IPv6 options.
- *
  * int (*parse_options)(const struct net_device *dev,
  *			struct nd_opt_hdr *nd_opt,
  *			struct ndisc_options *ndopts):
···
  *     addresses. E.g. 802.15.4 6LoWPAN.
  */
 struct ndisc_ops {
-	int	(*is_useropt)(u8 nd_opt_type);
 	int	(*parse_options)(const struct net_device *dev,
 				 struct nd_opt_hdr *nd_opt,
 				 struct ndisc_options *ndopts);
···
 };
 
 #if IS_ENABLED(CONFIG_IPV6)
-static inline int ndisc_ops_is_useropt(const struct net_device *dev,
-				       u8 nd_opt_type)
-{
-	if (dev->ndisc_ops && dev->ndisc_ops->is_useropt)
-		return dev->ndisc_ops->is_useropt(nd_opt_type);
-	else
-		return 0;
-}
-
 static inline int ndisc_ops_parse_options(const struct net_device *dev,
 					  struct nd_opt_hdr *nd_opt,
 					  struct ndisc_options *ndopts)

net/6lowpan/ndisc.c

···
 
 #include "6lowpan_i.h"
 
-static int lowpan_ndisc_is_useropt(u8 nd_opt_type)
-{
-	return nd_opt_type == ND_OPT_6CO;
-}
-
 #if IS_ENABLED(CONFIG_IEEE802154_6LOWPAN)
 #define NDISC_802154_SHORT_ADDR_LENGTH	1
 static int lowpan_ndisc_parse_802154_options(const struct net_device *dev,
···
 #endif
 
 const struct ndisc_ops lowpan_ndisc_ops = {
-	.is_useropt		= lowpan_ndisc_is_useropt,
 #if IS_ENABLED(CONFIG_IEEE802154_6LOWPAN)
 	.parse_options		= lowpan_ndisc_parse_options,
 	.update			= lowpan_ndisc_update,

net/ipv6/ndisc.c

···
 	return opt->nd_opt_type == ND_OPT_PREFIX_INFO ||
 		opt->nd_opt_type == ND_OPT_RDNSS ||
 		opt->nd_opt_type == ND_OPT_DNSSL ||
+		opt->nd_opt_type == ND_OPT_6CO ||
 		opt->nd_opt_type == ND_OPT_CAPTIVE_PORTAL ||
-		opt->nd_opt_type == ND_OPT_PREF64 ||
-		ndisc_ops_is_useropt(dev, opt->nd_opt_type);
+		opt->nd_opt_type == ND_OPT_PREF64;
 }
 
 static struct nd_opt_hdr *ndisc_next_useropt(const struct net_device *dev,