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>

scripts/coccinelle/misc/bugon.cocci

@@ -57,6 +57,6 @@ coccilib.org.print_todo(p[0], "WARNING use BUG_ON")
 p << r.p;
 @@
 
-msg="WARNING: Use BUG_ON"
+msg="WARNING: Use BUG_ON instead of if condition followed by BUG.\nPlease make sure the condition has no side effects (see conditional BUG_ON definition in include/asm-generic/bug.h)"
 coccilib.report.print_report(p[0], msg)