markers: add an if(0) to __mark_check_format()

Wrap __mark_check_format() into an if(0) to make sure that parameters such as

trace_mark(mm_page_alloc, "order %u pfn %lu", order, page?page_to_pfn(page):0);

(where page_to_pfn() has side-effects) won't generate code because of the
__mark_check_format().

Thanks to Jan Kiszka for reporting this.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: "Frank Ch. Eigler" <fche@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>