btrfs: use bool for return type of btrfs_block_can_be_shared()

Currently btrfs_block_can_be_shared() returns an int that is used as a
boolean. Since it all it needs is to return true or false, and it can't
return errors for example, change the return type from int to bool to
make it a bit more readable and obvious.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>