f2fs: spread f2fs_set_inode_flags()

This patch changes codes as below:
- use f2fs_set_inode_flags() to update i_flags atomically to avoid
potential race.
- synchronize F2FS_I(inode)->i_flags to inode->i_flags in
f2fs_new_inode().
- use f2fs_set_inode_flags() to simply codes in f2fs_quota_{on,off}.

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