xfstests: 276: new test to ensure btrfs backref integrity

xfstests: 276: new test to ensure btrfs backref integrity

This is a btrfs specific scratch test checking the backref walker. It
creates a file system with compressed and uncompressed data extents, picks
files randomly and uses filefrag to get their extents. It then asks the
btrfs utility (inspect-internal) to do the backref resolving from fs-logical
address (the one filefrag calls "physical") back to the inode number and
file-logical offset, verifying the result.

Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>
Signed-off-by: Christoph Hellwig <hch@lst.de>