For files smaller than 160 bytes, we can store them in spare inode space.
- It works only for unencrypted data.
- Enabled with flag inline_data
- Supported on 3.14 and beyond

We have to be sure to use fresh e2fsprog, but we went through that pain with ext4 crypto.

When enabled, we save 4K if the file is smaller than 160 bytes:
A file of 4 bytes: 
No using inline data:
stat /mnt/stateful_partition/unencrypted/cache/test 
  File: '/mnt/stateful_partition/unencrypted/cache/test'
  Size: 4               Blocks: 8          IO Block: 4096   regular file
Device: b301h/45825d    Inode: 1966110     Links: 1


Using it:
stat /mnt/stateful_partition/unencrypted/cache/test2
  File: '/mnt/stateful_partition/unencrypted/cache/test2'
  Size: 4               Blocks: 1          IO Block: 4096   regular file
Device: b301h/45825d    Inode: 1966111     Links: 1

Can be enable without reformating: newer file will get inlined when small.