backing: silence compiler warning using __printf

__printf marker was added in commit d2cc4dde ("bdi_register: add
__printf verification, fix arg mismatch") for function `bdi_register`
since it is useful to verify format and arguments. Apply equivalent gcc
attribute to `bdi_register_va`.

Remove warning triggered with W=1:

  mm/backing-dev.c:881:2: warning: function might be possible candidate for ‘gnu_printf’ format attribute [-Wsuggest-attribute=format]
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/backing-dev.h

@@ -28,6 +28,7 @@ void bdi_put(struct backing_dev_info *bdi);
 
 __printf(2, 3)
 int bdi_register(struct backing_dev_info *bdi, const char *fmt, ...);
+__printf(2, 0)
 int bdi_register_va(struct backing_dev_info *bdi, const char *fmt,
 		    va_list args);
 int bdi_register_owner(struct backing_dev_info *bdi, struct device *owner);