f2fs: add barrier mount option

This patch adds a mount option, barrier, in f2fs.
The barrier option is the opposite of nobarrier.
If this option is set, cache_flush commands are allowed to be issued.

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