xfs: use rtgroup busy extent list for FITRIM

For filesystems that have rtgroups and hence use the busy extent list
for freed rt space, use that busy extent list so that FITRIM can issue
discard commands asynchronously without worrying about other callers
accidentally allocating and using space that is being discarded.

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