Btrfs: separate DISCARD from __btrfs_map_block

Since DISCARD is not as important as an operation like write, we don't
copy it to target device during replace, and it makes __btrfs_map_block
less complex.
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>