lib/sizeof.h: typeas(): Add macro

lib/sizeof.h: typeas(): Add macro

This macro is like typeof(), but requires that the input is a type name.

This macro is useful because it parenthesizes the type name, which
allows one to create pointers to arbitrary types.  The following code
works for simple types:

	T  x;
	T  *p;

For example, that would work fine for 'int':

	int  x;
	int  *p;

However, that wouldn't work for types such as 'int[3]':

	int[3]  x;
	int[3]  *p;

But if we do instead

	typeas(T)  x;
	typeas(T)  *p;

now we can use 'int[3]' just fine:

	typeof((int[3]){})  x;
	typeof((int[3]){})  *p;

To understand this, we create a compound literal of type int[3], with
default values (zero, zero, zero), then get it's type, which is
obviously int[3].  And then we use that to declare a variable x of that
type, and a pointer p, which has type 'int(*)[3]'.

This would also work with something simpler.  One could use typeof(T)
directly:

	typeof(T)  x;
	typeof(T)  *p;

However, typeof() doesn't require that the input is a type; it accepts
arbitrary input.  The following is valid C:

	typeof(42)  x;

For our macro MALLOC(), where we want the second argument to be a type
name, we want to require that the argument is a type name.  For that, we
need to use typeas().

Reported-by: Alejandro Colomar <alx@kernel.org>
Suggested-by: Thiago Adams <thiago.adams@gmail.com>
Suggested-by: Martin Uecker <uecker@tugraz.at>
Signed-off-by: Alejandro Colomar <alx@kernel.org>