xfs: support the XFS_BTNUM_FINOBT free inode btree type

Define the AGI fields for the finobt root/level and add magic
numbers. Update the btree code to add support for the new
XFS_BTNUM_FINOBT inode btree.

The finobt root block is reserved immediately following the inobt
root block in the AG. Update XFS_PREALLOC_BLOCKS() to determine the
starting AG data block based on whether finobt support is enabled.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

diff --git a/fs/xfs/xfs_stats.c b/fs/xfs/xfs_stats.c
index ce372b7..f224038 100644
--- a/fs/xfs/xfs_stats.c
+++ b/fs/xfs/xfs_stats.c
@@ -59,6 +59,7 @@
 		{ "abtc2",		XFSSTAT_END_ABTC_V2		},
 		{ "bmbt2",		XFSSTAT_END_BMBT_V2		},
 		{ "ibt2",		XFSSTAT_END_IBT_V2		},
+		{ "fibt2",		XFSSTAT_END_FIBT_V2		},
 		/* we print both series of quota information together */
 		{ "qm",			XFSSTAT_END_QM			},
 	};