MAINTAINERS: Add f2fs's patchwork

>From now on, f2fs also has its own patchwork link, thanks to Jaegeuk
for starting this tool!

Let's update it to f2fs entry.

Signed-off-by: Yangtao Li <frank.li@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>