xfs: define locking primitives for realtime groups

xfs: define locking primitives for realtime groups

Source kernel commit: 0e4875b3fb24c5bfdf685876c76713cda5a23b65

Define helper functions to lock all metadata inodes related to a
realtime group.  There's not much to look at now, but this will become
important when we add per-rtgroup metadata files and online fsck code
for them.

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