btrfs: make fallback_to_cow take btrfs_inode

It really wants btrfs_inode and is prepration to converting
run_delalloc_nocow to taking btrfs_inode.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>