compiler.h, bug.h: Prevent double error messages with BUILD_BUG{,_ON}

compiler.h, bug.h: Prevent double error messages with BUILD_BUG{,_ON}

Prior to the introduction of __attribute__((error("msg"))) in gcc 4.3,
creating compile-time errors required a little trickery.  BUILD_BUG{,_ON}
uses this attribute when available to generate compile-time errors, but
also uses the negative-sized array trick for older compilers, resulting in
two error messages in some cases.  The reason it's "some" cases is that as
of gcc 4.4, the negative-sized array will not create an error in some
situations, like inline functions.

This patch replaces the negative-sized array code with the new
__compiletime_error_fallback() macro which expands to the same thing
unless the the error attribute is available, in which case it expands to
do{}while(0), resulting in exactly one compile-time error on all versions
of gcc.

Note that we are not changing the negative-sized array code for the
unoptimized version of BUILD_BUG_ON, since it has the potential to catch
problems that would be disabled in later versions of gcc were
__compiletime_error_fallback used.  The reason is that that an unoptimized
build can't always remove calls to an error-attributed function call (like
we are using) that should effectively become dead code if it were
optimized.  However, using a negative-sized array with a similar value
will not result in an false-positive (error).  The only caveat being that
it will also fail to catch valid conditions, which we should be expecting
in an unoptimized build anyway.

Signed-off-by: Daniel Santos <daniel.santos@pobox.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: David Rientjes <rientjes@google.com>
Cc: Joe Perches <joe@perches.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>