btrfs: zoned: pass start block to btrfs_use_zone_append

btrfs_use_zone_append only needs the passed in extent_map's block_start
member, so there's no need to pass in the full extent map.

This also enables the use of btrfs_use_zone_append in places where we only
have a start byte but no extent_map.

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