x86: uv io-apic - use BUILD_BUG_ON instead of BUG_ON

The expression is known to be true/false at compilation
time so we're allowed to use build-time instead of
run-time check. Also align 'entry' items assignment.

[ Impact: shrink kernel a bit, cleanup ]

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Jack Steiner <steiner@sgi.com>
LKML-Reference: <20090502093956.GB4791@lenovo>
Signed-off-by: Ingo Molnar <mingo@elte.hu>