Btrfs: add free space tree sanity tests

Btrfs: add free space tree sanity tests

This tests the operations on the free space tree trying to excercise all
of the main cases for both formats. Between this and xfstests, the free
space tree should have pretty good coverage.

Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>
Orabug: 26274676

(cherry picked from commit 7c55ee0c4afba4434d973117234577ae6ff77a1c)
Signed-off-by: Shan Hai <shan.hai@oracle.com>
Reviewed-by: Somasundaram Krishnasamy <somasundaram.krishnasamy@oracle.com>