memzero.[ch]: Define memzero() and strzero() as inline functions

memzero.[ch]: Define memzero() and strzero() as inline functions

There's no need to have these as macros, so use functions, which are a
lot safer: there's no need to worry about multiple evaluation of args,
and there's also more type safety.  Compiler warnings are also simpler,
as they don't dump all the nested macros.

Cc: Christian Göttsche <cgzones@googlemail.com>
Cc: Serge Hallyn <serge@hallyn.com>
Cc: Iker Pedrosa <ipedrosa@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>