Btrfs: Don't corrupt ram in shrink_extent_tree, leak it instead

Far from the perfect fix, but these structs are small.  TODO for the
next release.  The block group cache structs are referenced in many
different places, and it isn't safe to just free them while resizing.

A real fix will be a larger change to the allocator so that it doesn't
have to carry about the block group cache structs to find good places
to search for free blocks.
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/extent-tree.c

@@ -3437,8 +3437,10 @@ int btrfs_shrink_extent_tree(struct btrfs_root *root, u64 shrink_start)
 			   key.objectid, key.objectid + key.offset - 1,
 			   (unsigned int)-1, GFP_NOFS);
 
+	/*
 	memset(shrink_block_group, 0, sizeof(*shrink_block_group));
 	kfree(shrink_block_group);
+	*/
 
 	btrfs_del_item(trans, root, path);
 	btrfs_release_path(root, path);