xfs: add sysfs stats for zoned GC

xfs: add sysfs stats for zoned GC

Add counters of read, write and zone_reset operations as well as
GC written bytes to sysfs.  This way they can be easily used for
monitoring tools and test cases.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Hans Holmberg <hans.holmberg@wdc.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>