kernel: Implement statfs()

[alexlarsson]

This retargets statfs() to the first base directory so that we get at least some size info. Free space isn't super important as the fs is not writable, but it makes sense to have at least some value to avoid confusing apps thinging the disk is full.

Fixes https://github.com/containers/composefs/issues/18

Signed-off-by: Alexander Larsson [email protected]