xfs: add rtgroup-based realtime scrubbing context management

xfs: add rtgroup-based realtime scrubbing context management

Create a pair of helpers to deal with setting up the necessary incore
context to check metadata records against the realtime metadata.  Right
now this is limited to locking the realtime bitmap and summary inodes,
but as we add rmap and reflink to the realtime device this will grow to
include btree cursors.

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