btrfs: balance: factor out convert profile validation

The validation follows the same steps for all three block group types,
the existing helper validate_convert_profile can be enhanced and do more
of the common things.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>