xfs: reduce special casing for the open GC zone

xfs: reduce special casing for the open GC zone

Currently the open zone used for garbage collection is a special snow
flake, and it has been a bit annoying for some further zoned XFS work
I've been doing.

Remove the zi_open_gc_field and instead track the open GC zone in the
zi_open_zones list together with the normal open zones, and keep an extra
pointer and a reference of in the GC thread's data structure.  This means
anything iterating over open zones just has to look at zi_open_zones, and
the life time rules are consistent.  It also helps to add support for
multiple open GC zones if we ever need them, and removes a bit of code.

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