Use unlikely() in BUG_ON()/WARN_ON()

Use unlikely() in BUG_ON()/WARN_ON()

-fno-reorder-blocks was added in c/s 1712, when x86-64 just started to
become enabled. The reason it got added is entirely unclear to me, and
it prevents the intended effect of unlikely() constructs (in
particular
the ones added here) of moving out of line code which is expected to
never get executed, as well as using forward branches (which are
statically predicted taken by various processors' branch prediction
units) preferably to reach infrequently executed code.

Signed-off-by: Jan Beulich <jbeulich@novell.com>