Btrfs: increase the size of the free space cache

Btrfs: increase the size of the free space cache

Arne was complaining about the space cache having mismatching generation
numbers when debugging a deadlock.  This is because we can run out of space
in our preallocated range for our space cache if you have a pretty
fragmented amount of space in your pinned space.  So just increase the
amount of space we preallocate for space cache so we can be sure to have
enough space.  This will only really affect data ranges since their the only
chunks that end up larger than 256MB.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>