btrfs: test free space tree mount options

btrfs: test free space tree mount options

The behavior of different combinations of space_cache mount options
wasn't well defined, which led to a regression between my initial
patches adding the free space tree and the 4.5 release. Add a test to
exercise all of the meaningful permutations of clear_cache,
nospace_cache, and space_cache.

This is a regression test for Linux kernel commit "Btrfs: fix mount -o
clear_cache,space_cache=v2".

Signed-off-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>