xfs: add some rtgroup inode helpers

xfs: add some rtgroup inode helpers

Create some simple helpers to reduce the amount of typing whenever we
access rtgroup inodes.  Conversion was done with this spatch and some
minor reformatting:

@@
expression rtg;
@@

- rtg->rtg_inodes[XFS_RTGI_BITMAP]
+ rtg_bitmap(rtg)

@@
expression rtg;
@@

- rtg->rtg_inodes[XFS_RTGI_SUMMARY]
+ rtg_summary(rtg)

and the CLI command:

$ spatch --sp-file /tmp/moo.cocci --dir fs/xfs/ --use-gitgrep --in-place

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>