Revert "remove false BUG_ON() from run_posix_cpu_timers()"

This reverts commit 3de463c7.

Roland has another patch that allows us to leave the BUG_ON() in place
by just making sure that the condition it tests for really is always
true.

That goes in next.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>