xfs: support reserved blocks for the rt extent counter

The zoned space allocator will need reserved RT extents for garbage
collection and zeroing of partial blocks.  Move the resblks related
fields into the freecounter array so that they can be used for all
counters.

Co-developed-by: Hans Holmberg <hans.holmberg@wdc.com>
Signed-off-by: Hans Holmberg <hans.holmberg@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>