linux/stddef.h: Add sizeof_member() macro

At present we have 2 different macros to calculate the size of a member
of a struct: FIELD_SIZEOF() and sizeof_field(). As a prerequisite to
bringing uniformity to the entire kernel source tree, add sizeof_member()
macro as it is both more pleasant (not upper case) and more correct
(sizeof()-family cannot operate on bit fields; this is meant to operate
on struct members), as discussed[1].

Future patches will replace all occurrences of above macros with
sizeof_member().

[1] https://www.openwall.com/lists/kernel-hardening/2019/07/02/2



Signed-off-by: Pankaj Bharadiya <pankaj.laxminarayan.bharadiya@intel.com>
Link: https://lore.kernel.org/r/20190924105839.110713-2-pankaj.laxminarayan.bharadiya@intel.com


Co-developed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>