btrfs: zoned: reserve data_reloc block group on mount

btrfs: zoned: reserve data_reloc block group on mount

Create a block group dedicated for data relocation on mount of a zoned
filesystem.

If there is already more than one empty DATA block group on mount, this
one is picked for the data relocation block group, instead of a newly
created one.

This is done to ensure, there is always space for performing garbage
collection and the filesystem is not hitting ENOSPC under heavy overwrite
workloads.

CC: stable@vger.kernel.org # 6.6+
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>