Enable new features for nested overlayfs setups

Miklos,

Based on your feedback on v1 [1], following patches implement "nested xino", where "nested" doesn't have to be overlayfs over overlayfs. It can be overlayfs over another fs that publishes ino domain information, or another stacked fs aware of ino domains over overlayfs.

Series is available on my ovl-nested branch [2]. xfstests are also available on my github [3].
Note that the tests in group "overlay/exportfs.nested" already pass on current master.

The test group "overlay/union" uses the xfstests harness to run various unionmount-testsuite setups (e.g. samefs, non-samefs, non-samefs with some layers on same fs). This test group requires the symlink src/unionmount-testsuite to run the test suite.

Thanks,
Amir.

Changes since v1:
- "relax requirement for non null uuid" already merged
- Teach vfs about inode number domains
- Don't special case overlayfs as a lower fs

[1] https://marc.info/?l=linux-unionfs&m=154039437114960&w=2
[2] https://github.com/amir73il/linux/commits/ovl-nested
[3] https://github.com/amir73il/xfstests/commits/ovl-nested

Amir Goldstein (3):
vfs: introduce the concept of inode number domains
ovl: limit xino for some nested overlay cases
ovl: relax xino for some nested overlay cases

 fs/overlayfs/ovl_entry.h |   7 +-
 fs/overlayfs/super.c     | 200 ++++++++++++++++++++++++++++++---------
 fs/overlayfs/util.c      |   2 +-
 fs/super.c               |   2 +
 include/linux/fs.h       |  18 +++-
 5 files changed, 179 insertions(+), 50 deletions(-)

2.17.1