ip: officially support flag mngtmpaddr also for "ip addr del"

ip: officially support flag mngtmpaddr also for "ip addr del"

Kernel is being extended to support flag IFA_F_MANAGETEMPADDR also for
deletion of addresses. This will allow a userspace application to indicate
that for a global address the kernel should delete all related temporary
addresses as well.

"ip addr del" internally calls ipaddr_modify which silently accepts
any flag provided on the command line already, independent of the
actual command.
Therefore only the usage documentation needs to be extended.

Signed-off-by: Heiner Kallweit <heiner.kallweit@web.de>