overflow: Add struct_size_t() helper

overflow: Add struct_size_t() helper

While struct_size() is normally used in situations where the structure
type already has a pointer instance, there are places where no variable
is available. In the past, this has been worked around by using a typed
NULL first argument, but this is a bit ugly. Add a helper to do this,
and replace the handful of instances of the code pattern with it.

Instances were found with this Coccinelle script:

@struct_size_t@
identifier STRUCT, MEMBER;
expression COUNT;
@@

-       struct_size((struct STRUCT *)\(0\|NULL\),
+       struct_size_t(struct STRUCT,
                MEMBER, COUNT)

Suggested-by: Christoph Hellwig <[email protected]>
Cc: Jesse Brandeburg <[email protected]>
Cc: Tony Nguyen <[email protected]>
Cc: "David S. Miller" <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: Paolo Abeni <[email protected]>
Cc: James Smart <[email protected]>
Cc: Keith Busch <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Sagi Grimberg <[email protected]>
Cc: HighPoint Linux Team <[email protected]>
Cc: "James E.J. Bottomley" <[email protected]>
Cc: "Martin K. Petersen" <[email protected]>
Cc: Kashyap Desai <[email protected]>
Cc: Sumit Saxena <[email protected]>
Cc: Shivasharan S <[email protected]>
Cc: Don Brace <[email protected]>
Cc: "Darrick J. Wong" <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Guo Xuenan <[email protected]>
Cc: Gwan-gyeong Mun <[email protected]>
Cc: Nick Desaulniers <[email protected]>
Cc: Daniel Latypov <[email protected]>
Cc: kernel test robot <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>
Acked-by: Martin K. Petersen <[email protected]>
Reviewed-by: Darrick J. Wong <[email protected]>
Reviewed-by: Gustavo A. R. Silva <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Acked-by: Jakub Kicinski <[email protected]>
Reviewed-by: Alexander Lobakin <[email protected]>
Link: https://lore.kernel.org/r/[email protected]