BACKPORT: Add support for PIO p flag

draft-ietf-6man-pio-pflag is adding a new flag to the Prefix Information
Option to signal that the network can allocate a unique IPv6 prefix per
client via DHCPv6-PD (see draft-ietf-v6ops-dhcp-pd-per-device).

When ra_honor_pio_pflag is enabled, the presence of a P-flag causes
SLAAC autoconfiguration to be disabled for that particular PIO.

An automated test has been added in Android (r.android.com/3195335) to
go along with this change.

Cc: Maciej Żenczykowski <maze@google.com>
Cc: Lorenzo Colitti <lorenzo@google.com>
Cc: David Lamparter <equinox@opensourcerouting.org>
Cc: Simon Horman <horms@kernel.org>
Signed-off-by: Patrick Rohr <prohr@google.com>
Reviewed-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 990c3049)

Conflicts:
   Documentation/networking/ip-sysctl.rst
   include/linux/ipv6.h
   net/ipv6/addrconf.c

BUG=b:369964250
TEST=build

Change-Id: I1e7f787b6f1e0a5265953b0443ffc1223b799b17
Signed-off-by: Taoyu Li <taoyl@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/5895029


Reviewed-by: Patrick Rohr <prohr@google.com>
Reviewed-by: Hugo Benichi <hugobenichi@google.com>
Tested-by: Taoyu Li <taoyl@chromium.org>
Commit-Queue: Taoyu Li <taoyl@chromium.org>
Signed-off-by: Mateusz Urbańczyk <urbanczyk@google.com>