sizeof.h: Add SIZEOF_ARRAY() macro

sizeof.h: Add SIZEOF_ARRAY() macro

This makes it safe to call sizeof() on an array.  Calling sizeof()
directly on an array is dangerous, because if the array changes to be a
pointer, the behavior will unexpectedly change.  It's the same problem
as with NITEMS().

Link: <https://stackoverflow.com/a/57537491>
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>