net: ipv6: Add sysctl entry to disable MTU updates from RA

The kernel forcefully applies MTU values received in router
advertisements provided the new MTU is less than the current. This
behavior is undesirable when the user space is managing the MTU. Instead
a sysctl flag 'accept_ra_mtu' is introduced such that the user space
can control whether or not RA provided MTU updates should be applied.
The default behavior is unchanged; user space must explicitly set this flag
to 0 for RA MTUs to be ignored.

Change-Id: I9b2672d7c7804b6d5394516f451888d3ac8d7803
Signed-off-by: Harout Hedeshian <harouth@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>