[IPV4]: Reset scope when changing address

This bug did bite at least one user, who did have to resort to rebooting
the system after an "ifconfig eth0 127.0.0.1" typo.

Deleting the address and adding a new is a less intrusive workaround.
But I still beleive this is a bug that should be fixed.  Some way or
another.

Another possibility would be to remove the scope mangling based on
address.  This will always be incomplete (are 127/8 the only address
space with host scope requirements?)

We set the scope to RT_SCOPE_HOST if an IPv4 interface is configured
with a loopback address (127/8).  The scope is never reset, and will
remain set to RT_SCOPE_HOST after changing the address. This patch
resets the scope if the address is changed again, to restore normal
functionality.

Signed-off-by: Bjorn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>