btrfs: move btrfs_unlock_up_safe to other locking functions

The function belongs to the family of locking functions, so move it
there. The 'noinline' keyword is dropped as it's now an exported
function that does not need it.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>