btrfs: factor out check_removing_space_info() from btrfs_free_block_groups()

Factor out check_removing_space_info() from btrfs_free_block_groups(). It
sanity checks a to-be-removed space_info. There is no functional change.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>