f2fs: remove unneeded variable from f2fs_sync_fs

We can directly return '0' from the function, instead of introducing a
'ret' variable.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Amit Sahrawat <a.sahrawat@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>