ipv6: fix net.ipv6.conf.all interface DAD handlers

Currently, writing into
net.ipv6.conf.all.{accept_dad,use_optimistic,optimistic_dad} has no effect.
Fix handling of these flags by:

- using the maximum of global and per-interface values for the
  accept_dad flag. That is, if at least one of the two values is
  non-zero, enable DAD on the interface. If at least one value is
  set to 2, enable DAD and disable IPv6 operation on the interface if
  MAC-based link-local address was found

- using the logical OR of global and per-interface values for the
  optimistic_dad flag. If at least one of them is set to one, optimistic
  duplicate address detection (RFC 4429) is enabled on the interface

- using the logical OR of global and per-interface values for the
  use_optimistic flag. If at least one of them is set to one,
  optimistic addresses won't be marked as deprecated during source address
  selection on the interface.

While at it, as we're modifying the prototype for ipv6_use_optimistic_addr(),
drop inline, and let the compiler decide.

Fixes: 7fd2561e4e ("net: ipv6: Add a sysctl to make optimistic addresses useful candidates")
Signed-off-by: Matteo Croce <mcroce@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -1680,6 +1680,9 @@ accept_dad - INTEGER
 	2: Enable DAD, and disable IPv6 operation if MAC-based duplicate
 	   link-local address has been found.
 
+	DAD operation and mode on a given interface will be selected according
+	to the maximum value of conf/{all,interface}/accept_dad.
+
 force_tllao - BOOLEAN
 	Enable sending the target link-layer address option even when
 	responding to a unicast neighbor solicitation.
@@ -1727,16 +1730,23 @@ suppress_frag_ndisc - INTEGER
 
 optimistic_dad - BOOLEAN
 	Whether to perform Optimistic Duplicate Address Detection (RFC 4429).
-		0: disabled (default)
-		1: enabled
+	0: disabled (default)
+	1: enabled
+
+	Optimistic Duplicate Address Detection for the interface will be enabled
+	if at least one of conf/{all,interface}/optimistic_dad is set to 1,
+	it will be disabled otherwise.
 
 use_optimistic - BOOLEAN
 	If enabled, do not classify optimistic addresses as deprecated during
 	source address selection.  Preferred addresses will still be chosen
 	before optimistic addresses, subject to other ranking in the source
 	address selection algorithm.
-		0: disabled (default)
-		1: enabled
+	0: disabled (default)
+	1: enabled
+
+	This will be enabled if at least one of
+	conf/{all,interface}/use_optimistic is set to 1, disabled otherwise.
 
 stable_secret - IPv6 address
 	This IPv6 address will be used as a secret to generate IPv6