f2fs: fix to set noatime and immutable flag for quota file

We should set noatime bit for quota files, since no one cares about
atime of quota file, and we should set immutalbe bit as well, due to
nobody should write to the file through exported interfaces.

Meanwhile this patch use inode_lock to avoid race condition during
inode->i_flags, f2fs_inode->i_flags update.

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