btrfs: add test to stress chunk allocation/removal and fstrim

btrfs: add test to stress chunk allocation/removal and fstrim

Stress btrfs' block group allocation and deallocation while running
fstrim in parallel. Part of the goal is also to get data block groups
deallocated so that new metadata block groups, using the same physical
device space ranges, get allocated while fstrim is running. This caused
several issues ranging from invalid memory accesses, kernel crashes,
metadata or data corruption, free space cache inconsistencies, free
space leaks and memory leaks.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>