Btrfs: add mount -o inode_cache

This makes the inode map cache default to off until we
fix the overflow problem when the free space crcs don't fit
inside a single page.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/free-space-cache.c

@@ -2536,6 +2536,9 @@ int load_free_ino_cache(struct btrfs_fs_info *fs_info, struct btrfs_root *root)
 	int ret = 0;
 	u64 root_gen = btrfs_root_generation(&root->root_item);
 
+	if (!btrfs_test_opt(root, INODE_MAP_CACHE))
+		return 0;
+
 	/*
 	 * If we're unmounting then just return, since this does a search on the
 	 * normal root and not the commit root and we could deadlock.
@@ -2575,6 +2578,9 @@ int btrfs_write_out_ino_cache(struct btrfs_root *root,
 	struct inode *inode;
 	int ret;
 
+	if (!btrfs_test_opt(root, INODE_MAP_CACHE))
+		return 0;
+
 	inode = lookup_free_ino_inode(root, path);
 	if (IS_ERR(inode))
 		return 0;