btrfs: zoned: use filesystem size not disk size for reclaim decision

When deciding if a zoned filesystem is reaching the threshold to reclaim
data block groups, look at the size of the filesystem not to potentially
total available size of all drives in the filesystem.

Especially if a filesystem was created with mkfs' -b option, constraining
it to only a portion of the block device, the numbers won't match and
potentially garbage collection is kicking in too late.

Fixes: 3687fcb0 ("btrfs: zoned: make auto-reclaim less aggressive")
CC: stable@vger.kernel.org # 6.1+
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Tested-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>