xfs: error out when a superblock buffer update reduces the agcount

XFS currently does not support reducing the agcount, so error out if
a logged sb buffer tries to shrink the agcount.

Signed-off-by: Christoph Hellwig <[e-mail]>
Reviewed-by: Brian Foster <[e-mail]>
Reviewed-by: Darrick J. Wong <[e-mail]>
Signed-off-by: Carlos Maiolino <[e-mail]>

fs/xfs/xfs_buf_item_recover.c

···
 	 */
 	xfs_sb_from_disk(&mp->m_sb, dsb);
 
+	if (mp->m_sb.sb_agcount < orig_agcount) {
+		xfs_alert(mp, "Shrinking AG count in log recovery not supported");
+		return -EFSCORRUPTED;
+	}
+
 	/*
 	 * Initialize the new perags, and also update various block and inode
 	 * allocator setting based off the number of AGs or total blocks.