nilfs2: change license of exported header file

This allows other projects to carry copies of the header file related
to ABI and disk format (i.e. "nilfs2_fs.h") without it or distributors
having to worry about effects on the project's overall license terms.
It's also desired for switching the license of nilfs library to LGPL.

Jiro SEKIBA pointed out these license issues (Message-ID:
<87tylo7msw.wl%jir@sekiba.com>), and he suggested switching license of
the library and nilfs2_fs.h to GNU Lesser General Public License.  We
take in his suggestion to avoid the license issues.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Cc: Jiro SEKIBA <jir@unicus.jp>
Cc: linux-nilfs <linux-nilfs@vger.kernel.org>