xfs: enable FITRIM on the realtime device

xfs: enable FITRIM on the realtime device

Implement FITRIM for the realtime device by pretending that it's
"space" immediately after the data device.  We have to hold the
rtbitmap ILOCK while the discard operations are ongoing because there's
no busy extent tracking for the rt volume to prevent reallocations.

Cc: Konst Mayer <cdlscpmv@gmail.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>