f2fs: fix to avoid race when accessing last_disk_size

last_disk_size could be wrong due to concurrently updating, so using
i_sem semaphore to make last_disk_size updating exclusive to fix this
issue.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>