btrfs: remove super block argument from btrfs_iget_locked()

It's pointless to pass a super block argument to btrfs_iget_locked()
because we always pass a root and from it we can get the super block
through:

   root->fs_info->sb

So remove the super block argument.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>