btrfs: zoned: fix alloc_offset calculation for partly conventional block groups

When one of two zones composing a DUP block group is a conventional zone,
we have the zone_info[i]->alloc_offset = WP_CONVENTIONAL. That will, of
course, not match the write pointer of the other zone, and fails that
block group.

This commit solves that issue by properly recovering the emulated write
pointer from the last allocated extent. The offset for the SINGLE, DUP,
and RAID1 are straight-forward: it is same as the end of last allocated
extent. The RAID0 and RAID10 are a bit tricky that we need to do the math
of striping.

This is the kernel equivalent of Naohiro's user-space commit:
"btrfs-progs: zoned: fix alloc_offset calculation for partly
conventional block groups".

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