include/linux/kernel.h: add typeof_member() macro

Add typeof_member() macro so that types can be extracted without
introducing dummy variables.

Link: http://lkml.kernel.org/r/20190529190720.GA5703@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>