xfs: support growfs on zoned file systems

xfs: support growfs on zoned file systems

Replace the inner loop growing one RT bitmap block at a time with
one just modifying the superblock counters for growing an entire
zone (aka RTG).  The big restriction is just like at mkfs time only
a RT extent size of a single FSB is allowed, and the file system
capacity needs to be aligned to the zone size.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>