btrfs: compression: add helper for type to string conversion

commit e128f9c3f7242318e1c76d204c7ae32bc878b8c7 upstream.

There are several places opencoding this conversion, add a helper now
that we have 3 compression algorithms.

Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>