build_bug.h: more user friendly error messages in BUILD_BUG_ON_ZERO()

build_bug.h: more user friendly error messages in BUILD_BUG_ON_ZERO()

__BUILD_BUG_ON_ZERO_MSG(), as introduced in [1], makes it possible to
do a static assertions in expressions. The direct benefit is to
provide a meaningful error message instead of the cryptic negative
bitfield size error message currently returned by BUILD_BUG_ON_ZERO():

  ./include/linux/build_bug.h:16:51: error: negative width in bit-field '<anonymous>'
     16 | #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))
        |                                                   ^

Get rid of BUILD_BUG_ON_ZERO()'s bitfield size hack. Instead rely on
__BUILD_BUG_ON_ZERO_MSG() which in turn relies on C11's
_Static_assert().

Use some macro magic, similarly to static_assert(), to either use an
optional error message provided by the user or, when omitted, to
produce a default error message by stringifying the tested
expression. With this, for example:

  BUILD_BUG_ON_ZERO(1 > 0)

would now throw:

  ./include/linux/compiler.h:197:62: error: static assertion failed: "1 > 0 is true"
    197 | define __BUILD_BUG_ON_ZERO_MSG(e, msg) ((int)sizeof(struct {_Static_assert(!(e), msg);}))
        |                                                             ^~~~~~~~~~~~~~

Finally, __BUILD_BUG_ON_ZERO_MSG() is already guarded by an:

  #ifdef __CHECKER__

So no need any more for that guard clause for BUILD_BUG_ON_ZERO().
Remove it.

[1] commit d7a516c6eeae ("compiler.h: Fix undefined BUILD_BUG_ON_ZERO()")
Link: https://git.kernel.org/torvalds/c/d7a516c6eeae
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Link: https://git.kernel.org/next/linux-next/c/b88937277df
Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Yury Norov <yury.norov@gmail.com>