xfs: display more zone related information in mountstats

xfs: display more zone related information in mountstats

Modify xfs_zoned_show_stats() to add to the information displayed with
/proc/self/mountstats the total number of zones (RT groups) and the
number of open zones together with the maximum number of open zones.

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