reiserfs: Replace kmalloc with kcalloc in the comment

Use kcalloc instead of kmalloc in the comment according to
the previous kcalloc() call.

Link: https://lore.kernel.org/r/1590714150-15895-1-git-send-email-wang.yi59@zte.com.cn
Signed-off-by: Liao Pingfang <liao.pingfang@zte.com.cn>
Signed-off-by: Jan Kara <jack@suse.cz>