Fix macro API for __USE_KERNEL_IPV6_DEFS.

The use of __USE_KERNEL_IPV6_DEFS with ifndef is bad
practice per: https://sourceware.org/glibc/wiki/Wundef.
This change moves it to use 'if' and always define the
macro.

Please note that this is not the only problem with this
code. I have a series of fixes after this one to resolve
breakage with this code and add regression tests for it
via compile-only source testing (to be discussed in another
thread).

Unfortunately __USE_KERNEL_XATTR_DEFS is set by the kernel
and not glibc, and uses 'define', so we can't fix that yet.