fs: udf: Replace GPL 2.0 boilerplate license notice with SPDX identifier

The notice refers to full GPL 2.0 text on now defunct MIT FTP site [1].
Replace it with appropriate SPDX license identifier.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Pali Rohár <pali@kernel.org>
Link: https://web.archive.org/web/20020809115410/ftp://prep.ai.mit.edu/pub/gnu/GPL

 [1]
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20230522005434.22133-2-bagasdotme@gmail.com>