SAFE_ARRAY_SIZE() would return the size if there were no overflow
and -1 on errors?  We can't return zero on errors because there are
a lot of places which do zero size allocations and it's valid.  It
seems ugly.

I really think that's over thinking things.  Let's just match
kcalloc() exactly except without zeroing.  The BUILD_BUG_ON() thing
is an over complication as well.  We haven't needed it for
kcalloc().

The only impossible bit is picking the right name.

regards,
dan carpenter