freevxfs: relicense to GPLv2 only

When I wrote the freevxfs driver I had some odd choice of licensing
statements, the options are either GPL (without version) or an odd
BSD-ish licensense with advertising clause.

The GPL vs always meant to be the same as the kernel, that is version
2 only, and the odd BSD-ish license doesn't make much sense.  Add
a GPL2.0-only SPDX tag to make the GPL intentions clear and drop the
bogus BSD license.

Acked-by: Krzysztof Błaszkowski <kb@sysmikro.com.pl>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>