xfs: add frextents to the lazysbcounters when rtgroups enabled

xfs: add frextents to the lazysbcounters when rtgroups enabled

Source kernel commit: 35537f25d23697716f0070ea0a6e8b3f1fe10196

Make the free rt extent count a part of the lazy sb counters when the
realtime groups feature is enabled.  This is possible because the patch
to recompute frextents from the rtbitmap during log recovery predates
the code adding rtgroup support, hence we know that the value will
always be correct during runtime.

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