btrfs: fix ->free_chunk_space math in btrfs_shrink_device

There are two bugs in how we adjust ->free_chunk_space in
btrfs_shrink_device.  First we're removing the entire diff between
new_size and old_size from ->free_chunk_space.  This only works if we're
reducing the free area, which we could potentially not be.  So adjust
the math to only subtract the diff in the free space from
->free_chunk_space.

Additionally in the error case we're unconditionally adding the diff
back into ->free_chunk_space, which we need to only do if this device is
writeable.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>