btrfs-progs: tests: add back removed functionality

Previously the XFS-specific code was commented out so we don't need the
headers for building fsstress, this changes how the utility behaves
compared to the one in fstests, e.g. randomness or additional open/close
operations. Add enough code from xfsprogs to make it compile and enable
the #if0-ed code.

Signed-off-by: David Sterba <dsterba@suse.com>

Author: kdave