proc: use __auto_type more

Switch away from quite chatty declarations using typeof_member().

In theory this is faster to compile too because there is no macro
expansion and there is less type checking.

Link: https://lkml.kernel.org/r/81bf02fd-8724-4f4d-a2bb-c59620b7d716@p183


Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>