Now, in product, more and more features based on file encryption were
introduced, their demand of xattr space is increasing, however, inline
xattr has fixed-size of 200 bytes, once inline xattr space is full, new
increased xattr data would occupy additional xattr block which may bring
us more space usage and performance regression during persisting.

In order to resolve above issue, it's better to expand inline xattr size
flexibly according to user's requirement.

So this patch introduces new filesystem feature 'flexible inline xattr',
and new mount option 'inline_xattr_size=%u', once mkfs enables the
feature, we can use the option to make f2fs supporting flexible inline
xattr size.

To support this feature, we add extra attribute i_inline_xattr_size in
inode layout, indicating that how many space inline xattr borrows from
block address mapping space in inode layout, by this, we can easily
locate and store flexible-sized inline xattr data in inode.

Inode disk layout:
  +----------------------+
  | .i_mode              |
  | ...                  |
  | .i_ext               |
  +----------------------+
  | .i_extra_isize       |
  | .i_inline_xattr_size |-----------+
  | ...                  |           |
  +----------------------+           |
  | .i_addr              |           |
  |  - block address or  |           |
  |  - inline data       |           |
  +----------------------+<---+      v
  |    inline xattr      |    +---inline xattr range
  +----------------------+<---+
  | .i_nid               |
  +----------------------+
  |   node_footer        |
  | (nid, ino, offset)   |
  +----------------------+

Note that, we have to cnosider backward compatibility which reserved
inline_data space, 200 bytes, all the time, reported by Sheng Yong.

Previous inline data or directory always reserved 200 bytes in inode layout,
even if inline_xattr is disabled. In order to keep inline_dentry's structure
for backward compatibility, we get the space back only from inline_data.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Reported-by: Sheng Yong <shengyong1@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>