btrfs: drop unused parameter data from btrfs_fill_super()

[ Upstream commit 01c5db78 ]

The only caller passes NULL, we can drop the parameter. This is since
the new mount option parser done in 3bb17a25 ("btrfs: add get_tree
callback for new mount API").

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Stable-dep-of: 951a3f59 ("btrfs: fix mount failure due to remount races")
Signed-off-by: Sasha Levin <sashal@kernel.org>