btrfs: move btrfs_should_fragment_free_space into block-group.c

This function uses functions that are not defined in block-group.h, move
it into block-group.c in order to keep the header clean.

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