xfs: add rtgroup-based realtime scrubbing context management

Create a state tracking structure and helpers to initialize the tracking
structure so that we can check metadata records against the realtime
space management metadata.  Right now this is limited to grabbing the
incore rtgroup object, but we'll eventually add to the tracking
structure the ILOCK state and btree cursors.

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