xfs: delay initial open of the GC zone

xfs: delay initial open of the GC zone

The code currently used to select the new GC target zone when the
previous one is full also handles the case where there is no current GC
target zone at all.  Make use of that to simplify the logic in
xfs_zone_gc_mount.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hans Holmberg <hans.holmberg@wdc.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>