bug.h: prevent double evaulation of `condition' in BUILD_BUG_ON

bug.h: prevent double evaulation of `condition' in BUILD_BUG_ON

When calling BUILD_BUG_ON in an optimized build using gcc 4.3 and later,
the condition will be evaulated twice, possibily with side-effects.  This
patch eliminates that error.

[[email protected]: tweak code layout]
Signed-off-by: Daniel Santos <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Joe Perches <[email protected]>
Cc: Josh Triplett <[email protected]>
Cc: Paul Gortmaker <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>