The new ioctls to follow backrefs are not clean for 32/64 bit
compat.  This reworks them for u64s everywhere.  They are brand new, so
there are no problems with changing the interface now.
Signed-off-by: NChris Mason <chris.mason@oracle.com>

These helper functions iterate back references and call a function for each
backref. There is also a function to resolve an inode to a path in the
file system.
Signed-off-by: NJan Schmidt <list.btrfs@jan-o-sch.net>