xfs: add a xfs_qm_unmount_rt helper

RT group enabled file systems fix the bug where we pointlessly attach
quotas to the RT bitmap and summary files.  Split the code to detach the
quotas into a helper, make it conditional and document the differing
behavior for RT group and pre-RT group file systems.

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