MAINTAINERS: Add f2fs bug tracker link

As f2fs component in bugzilla.kernel.org was created and used since
2018-7.

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