f2fs: fix to set sbi dirty correctly

In order to record direct IO count, we add two additional type in
enum count_type: F2FS_DIO_{WRITE,READ}, but those IO won't dirty
filesystem metadata, so we don't need to set filesystem dirty in
inc_page_count(), fix it.

Fixes: 02b16d0a ("f2fs: add to account direct IO")
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>