scripts/coccinelle/misc/bugon.cocci: update bug_on conversion warning

if()/BUG conversion to BUG_ON must be avoided when there's side effect
in condition. The reason being BUG_ON won't execute the condition when
CONFIG_BUG is not defined.

Inspired-by: J. Bruce Fields <bfields@fieldses.org>
Suggested-by: Julia Lawall <Julia.Lawall@lip6.fr>
Acked-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Michal Marek <mmarek@suse.cz>