xfs: prevent gc from picking the same zone twice

When we are picking a zone for gc it might already be in the pipeline
which can lead to us moving the same data twice resulting in in write
amplification and a very unfortunate case where we keep on garbage
collecting the zone we just filled with migrated data stopping all
forward progress.

Fix this by introducing a count of on-going GC operations on a zone, and
skip any zone with ongoing GC when picking a new victim.

Fixes: 080d01c4 ("xfs: implement zoned garbage collection")
Signed-off-by: Hans Holmberg <hans.holmberg@wdc.com>
Co-developed-by: Hans Holmberg <hans.holmberg@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Tested-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>