While the numerical values of IFA_F_SECONDARY and IFA_F_TEMPORARY
are identical, their meaning is not.

IFA_F_SECONDARY is only relevant for IPv4 addresses, while
IFA_F_TEMPORARY is only relevant for IPv6 addresses.

IFA_F_TEMPORARY is automatically set by kernel for the addresses
that it generates as part of IFA_F_MANAGETEMPADDR. It cannot be
actively set by user-space.

IFA_F_SECONDARY is automatically set by kernel depending on the order
in which the addresses for the same subnet are added.

This essentially reverts 8b4f1192 (core: avoid IFA_F_TEMPORARY alias for
IFA_F_SECONDARY).