f2fs: introduce private inode status mapping

Previously, we use generic FS_*_FL defined by vfs to indicate inode status
for each bit of i_flags, so f2fs's flag status definition is tied to vfs'
one, it will be hard for f2fs to reuse bits f2fs never used to indicate
new status..

In order to solve this issue, we introduce private inode status mapping,
Note, for these bits have already been persisted into disk, we should
never change their definition, for other ones, we can remap them for
later new coming status.

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